Messerschmitt Bf.110 in service with the Luftwaffe. Part seven. Bf.110E and the Bf.110 in North Africa.

 

After the Battle of Britain revealed the mediocre performance of the Bf.110 as a heavy fighter, there were doubts about its most efficient role, specially when its replacement, the Messerschmitt Me.210, was already expected soon. 
However, the Me.210's programme was plagued with delays and problems and given that there was nothing better, it was decided to keep the Bf.110 under production. 
After that decision, the Bf.110 was assigned a new role: Fighter-Bomber, making it basically a Bf.110D with various bomb racks under wings and fuselage. 
Manufactured together with the Bf.110D, the only visible difference between both versions was the presence of a small rectangular air intake on the nose between the two upper machine guns. This opening was intended for the cockpit heating. The Bf.110E's tailwheel was also larger than previous models, as well as the shock absorber. Pitot tube was also lengthened and moved below the starboard wing leading edge, beyond the engine nacelle and the rudder trim tab was enlarged and now overlapped the trailing edge. 
The rear tip of the fuselage could be fitted with an additional fairing containing a life raft and it could be tropicalised very easily too with extra sand filters and larger oil radiators. It was also included improved nose gun ports and a survival kit with water, food and small weapons for the crew inside the cockpit. 
Just like its predecessors, it was brought out in several variants, identified by one or more suffixes added to the original designation.

In the North African campaign, things were not going well for the Italians in late 1940, who were in full retreat against British forces. In order to prevent full control of Africa by the British, the Germans dispatched forces to prop up Mussolini's troops, at least enough to keep them from collapsing. 
This support was materialized in December 1940 when III./ZG 26 was sent to the Mediterranean and was based at Trapani, Sicily, where it carried out many types of missions such as escort fighters for planes or ships, anti-shipping patrols, watching over air and sea routes between Italy and North Africa and also aerial raids over Malta.
The long range of the Bf.110 allowed them to be dispersed all along the Mediterranean in Italy, Crete or North Africa.
The first of those aerial units to arrive in north Africa was the Bf.110-equipped III./ZG 26 commanded by Major Karl Kaschka, which was reinforced by the 2. Staffel, arriving on 30th January 1941 and were immediately dispersed to airfields at Castel Benito, Sirte and Arco Philenorum, in Libya. Put quickly into action, the Zerstörer units lost six aircraft by the end of February, having shot down only two enemy machines.
At the end of March Rommel attacked, leading his forces against the British lines and forcing them to withdraw and beginning a retreat that threatened Egypt. Given the vast extensions they had to fly in the north African desert, the Bf.110 coped with the range problem much better than it did during the Battle of Britain and by late August 1941 III./ZG 26 claimed 33 Hawker Hurricanes and Tomahawks (P-40C), which were their main foe, for the lost of 23 Zerstörer.

Sources:
1st Signal Squadron - Aircraft In action  1030 - Messerschmitt Bf-110 Zerstörer in Action
2nd Histoire & Collections - Avions et Pilotes 11 - Messerschmitt Bf.110 & Me.210-410 1939 -1945
3rd https://en.wikipedia.org/wiki/Messerschmitt_Bf_110_operational_history

Messerschmitt Bf.110 in service with Croatia, Hungary and the USSR

 

The Messerschmitt Bf.110 was a German twin-engined heavy fighter of the Second World War that was employed by various countries, among them, the following ones:

• Independent State of Croatia: Very late in the war, in February 1945, two Bf.110G-4 night fighters were supplied to the Independent State of Croatia's Air Force (ZNDH for short). One of them was destroyed during an Allied bombing in Zagreb, while the another retreated to Klagenfurt, Austria, fleeing from the advance of Yugoslav partisans and the Soviet Army. Its fate is unknown.
• Hungary: The Royal Hungarian Air Force (MKHL for short) was supplied with approximately twelve Bf.110G-4 night fighters in December 1944 (although some sources claim there were up to fourteen). 
  They were assigned to the 101. Night Fighter Squadron, with plans set to fund a second one. In fact, pilots were trained for this second unit, but never received the required aircraft. 
  The 101 was initially based in western Hungary but given the quick advance of the Soviet Army, they were soon rebased to Vienna and Parndorf. In the period ranging from January to March 1945 they saw barely any action at all flying some unfruitful sorties. In early April 1945 their aircraft were blown up on ground to prevent their capture as they could not fly due to the lack of fuel. After that, the crews moved to  southern Germany to surrender to American troops and avoid being taken prisoners by the Soviets.
• USSR: The Soviet Air Force (VVS) bought a single Bf.110C in autumn 1940 for evaluation purposes. It was tested by the Aeronautical Institute of the VVS and served as the main inspiration for the Petlyakov Pe-3.

Sources:
1st https://en.wikipedia.org/wiki/Messerschmitt_Bf_110_operational_history
2nd https://falkeeins.blogspot.com/2015/06/hungarian-bf-110-g-4-101-night-fighter.html
3rd Hikoki Publications - The Croatian Air Force

4th Schiffer Publications -  German Aircraft in Russian and Soviet Service 1914-1951 Volume I 

Messerschmitt Bf.110 in service with the Luftwaffe. Part six. Bf.110 in Operation Barbarossa.

 

Just before the start of Operation Barbarossa, the German invasion of the USSR on 21st June 1941, one Zerstörer group,  II./ZG 26 was sent to Suwalki, in Poland. The approximately fifty aircraft, out of a force of almost 2.500 German aircraft, took part from the very beginning in the campaign, but encountered very few resistance, since the Russians had a widely outdated air force, which was, for the most part, destroyed on the ground. 
In fact, the first week of Barbarossa brough the Luftwaffe an unbelievable success, with claims mounting to over 6.000 Soviet aircraft destroyed and, although those claims were exaggerated, it's known that the Soviet Military Air Power Commander, Lt.-Gen. Kopels committed suicide on the second day of Barbarossa when he got the news that 600 were already lost for just 12 German airplanes. 
Since most of the Zerstörer-equipped Gruppen (a Luftwaffe unit which is the rough equivalent to a squadron) that survived the Battle of Britain were either in North Africa, providing aerial support for Rommel or were being retrained for night-fighting roles in order to fight the increasing Royal Air Force's  night raids on Germany, there were only two Gruppe which could be employed in Barbarossa: Schnellkampfgeschwader 210 (SKG 210) and II./ZG 26.
Attached to Luftflotte 2 and providing support for Army Group Center, the Bf.110, struck Soviet airfields in low level attacks. Finding the Russian airfields covered with aircraft parked in peacetime rows without camouflage, the Zerstörers straked across their assigned airfields bombing and strafing everything in sight leaving only destroyed aircraft and airfield installations behind. During the first wave the Luftwaffe attacked 31 airfields, destroying hundreds of Soviet machines and losing only two aircraft. The attacks went on hour after hour and day after day with complete Soviet air units being destroyed on the ground and the few machines that could get into the air were quickly dealt with. Because most of the destruction took place on the ground, however, few aircrew were lost in comparison, leaving the Russian a large pool of personnel with which to begin rebuilding.

After the first few weeks the VVS (Soviet Air Force) could only offer a very weak resistance and the Bf.110s switched to low-level attacks, carrying out strafing and bombing attacks on front lines, roads, railway lines and supply hubs. By 31st July 1941 ZG claimed a total of 620 enemy aircraft destroyed on the ground and aerial combats, however, those claims are, were already wrote, unconfirmed and are most probably propaganda.
On 19th August, ZG 26 was officially noted for performing two attack on a Soviet airbase at Nizino, south of Leningrad, destroying 15 and leaving 30 burning on the ground plus 3 more machines being destroyed in the air, raising their score to 854 enemy machines, including 191 in aerial combat. On 15th September Operations Beowulf I and II took place, in which II./ZG 26. Those consisted in the amphibious assaults on the islands of Saaremaa, Hiiumaa and Muhu off the coast of Estonia. This time the Bf.110s acted as a flying artillery and performed a total of 118 sorties losing only two machines. 
Between 22nd June and 27th September ZG 26 was credited with 96 enemy aircraft shot down, 741 aircraft destroyed on ground, 148 tanks destroyed, 166 artillery guns, 3,280 vehicles of all kind, 49 trains (one of them armoured), 68 locomotives and 4 bridges.

In the northern sector two units operated the Bf.110: II./ZG 76 and I/JG 77 under the command of Luftflotte 5. Based at Oslo, Norway, it wasn't until the 25th June that they began their operations because of bad weather. On that same day they attacked the airfields at Niva, Varlamova and Vaenga, located in northern Russia. In those raids the German Zerstörers managed to catch the Russian aircraft. They were rebased to Finland, in order to shorten the distance to Murmansk and raiding enemy positions every time the weather permitted. Sometimes the Bf.110s were also employed as escort fighters for the bombers. By 12th July Hauptmann (Captain) G. Schascke claimed 12 kills and by 4th August he achieved his 20th kill, but was shot down behind Soviet lines. By fall 1941, however ZG 76 was rebased to Germany to be converted to the night-fighter role, leaving only 7.(Z)/JG 5 operating twelve Bf.110 within Luftflotte 5.

In the southern sector both I. and II./SKG 210 (former Erprobungsgruppe 210) flew ground attack missions in support of Operation Taifun, an envelope attack aimed to destroy Soviet Army Group Timoshenko and open a way to Moscow. This huge pincer movement was completed by 7th October, but the advance towards Moscow was hampered by rain and the ensuing mud. By 13th November, cold had set in and the Panzer divisions were ready to operate again accross the hardened ground toward Moscow and by the 27th November elements of the 3rd Panzergruppe reached the Volga canal only 30.5 km (19 miles) away from the centre of Moscow. The Russian winter began to take a toll on the Wehrmacht, which was not prepared for that situation, with bitter cold and heavy snows and, on 5-6 December, just when the Germans switched from the offensive to defensive, the Soviets struck and launched a massive counter-attack pushing the Germans back from Moscow. After a hard delaying battle, the Germans succeeded in establishing a new defensive, albeit thin in some places.
In late 1941 production of Bf.110 was still undergoing and the whole year saw a limited rise in the success rate of the Zerstörer, as we just saw.

Sources:
1st Signal Squadron - Aircraft In action  1030 - Messerschmitt Bf-110 Zerstörer in Action
2nd Histoire & Collections - Avions et Pilotes 11 - Messerschmitt Bf.110 & Me.210-410 1939 -1945
3rd https://en.wikipedia.org/wiki/Messerschmitt_Bf_110_operational_history

Messerschmitt Bf.110 in service with the Luftwaffe. Part five. Messerschmitt Bf.110D, the Balkans and Greece.

 

In order to keep with the idea of a heavy fighter capable of penetrating deep into enemy territory, the Reichluftfahrtministerium (Reich Air Ministry - RLM for short) gave priority to increase the Bf.110's range, with this new version called Bf.110D 'Dora'. 
The first sub-variant, D-0 was equipped with an extra 1,050L  (230 gal) fuel tank, together with a 106L (23 gal.) oil tank. Both were set inside a canvas and plywood fairing, whose shape earned it the nickname of Dackelbauch (German word for Dachshund's belly). 
Those additional tanks were integrated within the fuselage and could not be dropped, making the type even bulkier and less manoeuvrable. It also became the plane's weak spot because, due to its poor ventilation, a very inflammable mixture of air and patrol vapours were accumulated inside, giving way to many incidents and accidents. 
This long range version saw service first with V(Z)./LG 1 during spring 1940. Given its longer tail-end containing a two-seat inflatable raft, the Bf.110D was immediately recognisable. 
The Luftwaffe wanted this version to perform more tasks, so in April 1940 Messerschmitt was tasked to work on a variant equipped with a streamlined rectangular fairing under the fuselage, hosting two bomb hardpoints. This variant, named as Bf.110D-0/B (the standing for bomber) was stripped of the bulky fuel tanks and was supplied to Erprobungsgruppe 210, an experimental unit created at Cologne in July 1940, which was put into action in August that same year.
The extra fuel tank originated some problems, which gave way to the D-1 variant, a long-range fighter which had two 900L (200 gal) drop tanks under each wing with an additional oil tank under the fuselage in an fairing. The D-2 variant, was a direct derivation of the D-1 and was intended for the fighter-bomber role. It could carry two 500kg (1,100-lb) SC 500 bombs under the fuselage as well as two 300L (66 gal) drop tanks under the wings. Both variants, D-0/B and D-2 were scarcely manufactured with only 70 examples being built. 
The last version of the Dora was the D-3, which was the same as previous models but had not attachment equipment for the fuel tanks.

Since the end of the Battle of Britain, the Zerstörerwaffe was almost dismembered and its groups transferred and assigned to new tasks, mainly nigh-fighting and coastal surveillance along German, Norwegian and Sicilian shores. However, as new fronts were opening in central and eastern Europe, the Luftwaffe had to re-form the Zerstörer units, especially in areas where the opposition was weak or where long-range aircraft were needed.
The Bf.110 re-appeared en masse during the invasion of Yugoslavia and Greece, in operations prior to Operation Barbarossa, the surprise attack on the USSR. 
For operate in the Balkans, Luftflotte 4 requested long-range fighters and received two Zerstörer groups, I. and II./ZG 26, which were previously assigned to night-fighter units and were transferred to Bulgaria, where they conducted various attack operations against Yugoslavia in April 1941. Short after that invasion kicked-off, they were joined by the last Gruppe of the geschwader, III./ZG 26, which was based in Italy. 
When Yugoslavia was defeated and its air force neutralized by quick aerial strikes on their main airfields, the front shifted to Greece, which was also defeated fast in spite of reinforcements of the Royal Air Force.
The final fighting took place on Crete, where the Zerstörers of a newly found unit, II./ZG 76 fought alongside I. and II./ZG 26. On this occasion the Bf.110s were employed for strafing attacks and bombing land positions or even anti-shipping duties.

Sources:
1st Histoire & Collections - Avions et Pilotes 11 - Messerschmitt Bf.110 & Me.210-410 1939 -1945

2nd https://en.wikipedia.org/wiki/Messerschmitt_Bf_110_operational_history

Messerschmitt Bf.110 in service with the Luftwaffe. Part four. Bf.110 during the Battle of Britain and beyond.

 

This post is a direct continuation of this one:
Messerschmitt Bf.110 in service with the Luftwaffe. Part three. Blitzkrieg on the North & the West.
Barely one year after the beginning of its fighting career, the Bf.110 began to show its defects. After months of mixed results, the concept of the of the Zerstörer had to be seriously rethought, if not discarded entirely.
Just one few weeks after the fall of France, in early July 1940 the Third Reich began with what they considered the final stage of their conquest of Western Europe: the Operation Sealion, codename for the Invasion of England. This was expected to be costly both in men and materiel, it was decided to make sure that the Luftwaffe had the aerial superiority over British skies before the land forces were sent against the south-eastern coast of England. The plan comprised first reducing the sea traffic to nothing in the Channel and the North Sea and then destroying British airfields with massive air raids, employing conventional or dive bombers, under the protection of single-seat or twin-engined fighters.
Taking that into consideration, on 9th July 1940 General Kesselring, commander of Luftflotte 2, whose zone of operations ranged from the Netherlands to Normandy, decided to throw his Zerstörer groups (IE, the Bf.110-equipped units) into the battle for the first time since the Battle of France, with 220 Bf.110Cs being available at that moment. 
On that same day, a naval convoy was spotted entering the Channel so it was decided to intercept it using a large formation of bombers protected by Bf.110Cs. After some chaotical fighting due to bad weather conditions, four the of the Bf.110s were shot down by Royal Air Force's (RAF) Hurricanes.
Even if the loss ratios were not abysmal (the RAF lost the same number of planes during the initial engagements), the Zerstörers failed in their assigned role and showed again how vulnerable they were when they had to face a more manoeuvrable enemy. 
The different Bf.110 units engaged in the Battle of Britain were more or less successful in the many clashes that took place over Sussex and Kent. 
Often at their own cost the Bf.110 pilots had to cope with the shortcomings of their own machines almost on a daily basis, such as sluggish acceleration and low top speed which prevented them from breaking off formation, when necessary to fight. This was emphasized when facing both the Hawker Hurricane and the Supermarine Spitfire.
In order to make the Bf.110 less vulnerable, a new tactic was developed over Dunkirk during the closing stages of the Battle of France and was employed to good effect. This consisted on the Bf.110s forming a defensive circle if they were attacked, with the Zerstörers circling in line astern at the same altitude, protecting each other from enemy attacks with both their defensive and offensive weapons. However, this meant that an aircraft designed mainly as an attack plane couldn't perform offensive actions, but defensive one. 
Hermann Göring, the creator of the Zerstörer concept, was personally affected by the initial losses, since his own nephew, Hans-Joachim, a pilot assigned to III./ZG 76 was shot down by Hurricanes on 11th July 1940 over Weymouth Bay.

The Luftwaffe almost succeeded in closing the Channel to all maritime shipping and therefore, moved on to the second phase on the Attack on Britain, called Adlertag (Eagle Day), which took place on 13th August. On that day a massive aerial raid took place, with the Bf.110s claiming approximately thirty kills at the cost of thirteen of their own machines, which made Göring furious. However, the worst day for the Bf.110 during the Battle of Britain was 15th August, when losses topped to thirty planes shot down or damaged beyond repair. Thirty aircraft was the equivalent of a Gruppe for the Luftwaffe. 
To make things worse, three experimented Gruppenkommandeurs (Group Commanders - the equivalent to Wing Commanders in the RAF) were killed during the various engagements when a group of approximately twenty Bf.110D-s based in Norway took part in the attacks; they could be recognized by their extra ventral fuel tank which hampered their manoeuvrability, even if their range was longer. One third of those D-1s which took part in the raid were shot down by British fighters which considered them an easy prey. 
During the following days of August the situation got only worse, specially for ZG 76, the main unit operating Bf.110s as their casualties kept on rising at an unrelenting rate. On 17th August ZG 76 alone lost fifteen machines, totalling 120 machines by the end of the month.
On 18th August Bf.110 activity began to decline for the simple reason that the Luftwaffe couldn't make up the losses they were suffering and the number of available machines was starting to be dangerously low.
Despite all this, Göring, recently promoted to Reichsmarschall, decided to keep the Bf.110s in frontline service by assigning them to missions not suitable for single-engined fighters: those deep inside British territory. With the Luftwaffe still not having enough single-seat fighters, the Bf.110s still available were thrown into the Battle of London, which was part of the second stage of the Battle of Britain, with dramatic results as expected. 
Furthermore, nothing could be done to improve the situation, not even decorating certain Bf.110 unit commanders to boost morale in those seriously beaten units.
On 4th and 27th September a total of fifteen machines were lost, serving with the Erprobungsgruppe 210 (another unit operating the Bf.110), losing on this occasion its third Commanding Officer in as many weeks. When Sealion was finally discarded, 223 out of a total of 237 operational Bf.110s were lost. 
The end of the Battle of Britain sealed the fate of Göring's Zerstörerwaffe concept and the Bf.110 was withdrawn from frontline units, which were either disbanded, just like ZG.2 in October 1940 or sent to the rearguard, like ZG.26 (which lost seven machines on 7th October) or ZG.76.
Just like it happened to the Stuka, the other Luftwaffe icon of the Blitzkrieg, it was decided to send the Bf.110 to the other safer theatres of operations, and the Bf.110 no longer saw daytime combat on the Western Front.

Sources:
1st Histoire & Collections - Avions et Pilotes 11 - Messerschmitt Bf.110 & Me.210-410 1939 -1945

2nd https://en.wikipedia.org/wiki/Messerschmitt_Bf_110_operational_history

Curtiss P-36. Part Eighteen. The P-36 in Pearl Harbor.

 

This is our last post before our summer vacations. See you back later!
The P-36 was delivered to Hawaii in February 1941 on board USS Enterprise (CV-6) aircraft carrier. The Hawks were loaded in California, and then flown off the carrier's deck when the Enterprise was close the shore of Hawaii. This kind of procedure was the first of its kind for the United States Army Air Corps (USAAC) and would later be replicated during World War 2 many times, like during the Siege of Malta, for example. 
This procedure saved considerable time and resources compared to the traditional methos of having the fighters disassembled first, crated and then loaded by crane in a freighter ship, then unloaded and reassembled in Hawaii. 
The only combat action saw by the P-36 in USAAC's markings took place on 7th December 1941, during the bombing of Pearl Harbor. 
During the second wave of Pearl Harbor, amidst the chaos at Wheeler Field, a total of four P-40Cs and two P-36s took off at 8:35 hours in the morning, this was preceded by two P-40s and four P-36 which took off five minutes earlier. One of those who took off flying a P-36 was 2 Lt. Harry W. Brown who took off from Haleiwa auxiliary field and, when headed for Wheeler he soon spotted another P-36 flown by 2 Lt. Bob Rogers and they both managed to shot down a Japanese Nakajima B5N 'Kate' torpedo-bomber of the Imperial Japanese Navy (IJN).  2 Lt. Brown would eventually become an ace and that marked his only aerial victory flying a P-36. After that, he landed back at Haleiwa where the other Curtiss fighters also landed to refuel.
At Wheeler 1 Lt. Lewis Sanders, managed to save many P-36s and got them ready for action. As he was the commander of the 46th Pursuit Squadron, he led four P-36s off at 8:50 am just when the second wave began its attack. One fighter was flown by 2 Lt. Philip Rasmussen who, given the urgence of the situation, had to fly wearing his pyjama and took off in an all-silver P-36A. Among other pilots was 2 Lt. Gordon Sterling who, in the chaos and confusion, manned 2 Lt. Norris' aircraft.
Most of pilots from Wheeler and Haleiwa had just passed their gunnery tests and were, theoretically, better qualified than many other pilots in Hawaii at that time.
In order to gain height advantage Sanders climbed hard for the first 20 minutes of the mission and then he attacked the leading Mitsubishi A6M Zero flown by Lt. Fujita, while 2 Lt. Thacker and Rasmussen were engaged with other IJN fighters. Meanwhile, Sterling managed to get behind another Zero, but got himself in trouble when Fujita's Zero slipped on his tail. Seeing his wingman under fire, Sanders dived on Fujita but couldn't save Sterling who died.
Both Sanders and Sterling were credited with victories, although both Japanese fighters survived, albeit with damage.
While all that combat was taking place, 2. Lt. Thacker experienced a beam attack from a Zero when he was flying in circles trying to re-cock his guns. This attacked damaged his P-36 and the Zero, thought to be flown by P.O. 2c Tanaka, then crossed the nose of 2 Lt. Rasmussen's P-36. In the ensuing fight, the result was uncertain, but Rasmussen remembered to have seen the Zero smoking.
Back at Wheeler, 2 Lt. Bill Haney (future ace) ran to a line of P-36 that were being armed, when the bombs were falling he managed to take off and join Sanders' small ad-hoc group of fighters, only to be struck by friendly anti-air fire coming from the heavy cruiser USS San Francisco. With his engine damaged Haney was forced to land. Just when he landed he sprinted towards another P-36 and took-off only to be hit another friendly anti-air fire which forced him to land again back at Wheeler field. 
When the second wave retreated to the sea, seven P-36s and a pair of P-40s took off to chase them, however they failed to intercept the IJN fighters and flew an uneventful patrol. 
The combat career of the P-36 with the USAAC lasted less than one hour, as the type was outdated and they were quickly replaced by the P-40 Warhawk or other types. By the Battle of Midway in June 1942 the 18th Pursuit Squadron based at Elmendorf Field, Alaska had only three exampled on strength, those being potentially the last American P-36 that saw some frontline action against the Japanese (although they just were employed for uneventful patrols over the southern shore of Alaska), as other USAAF's P-36 were still serving in frontline service, but were based in Panama Canal Zone with the 16th and 32nd Pursuit Groups, but experienced no combat.
During the year 1942 almost every P-36 was relegated to second line duties such as schools or used for mechanics training.

The Curtiss XP-42 was an experimental fighter, developed directly from the P-36. It was powered by a Pratt & Whitney R-1830 Twin Wasp engine and was fitted with a longer and streamlined cowling and a large propeller spinner, in order to improve the aerodynamics of the radial engine.
It flew for the first time in March 1939 and, although faster than the P-36, the P-40 proved to be faster and the new nose arrangement proved to be unresolvable in spite of at least 12 modifications, which eventually resulted in a shorter and less streamlined cowling closely resembling the original P-36. The XP-42 project was cancelled, however the prototype was retained by Curtiss and used as a test bed and was fitted with an all-moving tail. This aircraft was scrapped on 15th July 1947.

Sources:
1st https://en.wikipedia.org/wiki/Curtiss_P-36_Hawk#United_States
2nd Osprey - Aircraf of the Aces 86 - P-36 Hawk Aces of World War II
3rd https://en.wikipedia.org/wiki/Curtiss_XP-42

Curtiss P-36. Part Seventeen.

 

Following with the experimental P-36 variants, the XP-36E was a regular P-36A up-armed with four .30in machine guns plus the two fuselage ones, the XP-36F was armed with two 23 mm (0,91 in) Madsen autocannons placed under the wings, however it was reverted back to regular P-36A's configuration because the cannons created an unacceptable top-speed penalty of just 265 mph (426 km/h).
The XP-36D replaced its fuselage machine guns with two .50 in (12,7 mm) and had four .30 in machine guns in the wings. 
The XP-36B featured an up-rated R-1830-25 engine rated at 1,100 hp which only achieved a top speed of 313 mph (504 km/h), as it was marginally better, it was reverted back to the P-36A. 
All those experimental versions were carried out 1939 and early 1940 and served to test various armament arrangements, however, none of them gave acceptable results and none was fully developed.
The P-36 also received some unusual camouflage scheme. Shown at the Cleveland Air Races in September 1939, and flown by the 27th Pursuit Squadron this scheme consisted on various mix-and-match colours, intended to be disruptive. 

As stated, the P-36 was employed by various Pursuit Groups of the United States Army Air Corps (USAAC), among them 1st Pursuit Group (PG) which employed the type together with the Seversky P-35, until both were replaced by the Republic P-43 Lancer in 1940.
Another unit that the P-36 saw service with was 8th Pursuit Group which, just like 1st PG, employed the P-36 together with the Seversky P-35 during mid-to-late 1930s, which were replaced by the Curtiss P-40 in 1940.
A non-combat unit that employed the P-36 was the 36th Training Group which was activated on 1st February 1940 at Langley Field, Virginia and was initially equipped with the P-36 until being rebased to Puerto Rico in January 1941 and re-equipped with the Bell P-39 Airacobra and the Curtiss P-40.

Sources:
1st https://en.wikipedia.org/wiki/Curtiss_P-36_Hawk
2nd https://en.wikipedia.org/wiki/1st_Operations_Group
3rd https://en.wikipedia.org/wiki/8th_Operations_Group 
4th https://en.wikipedia.org/wiki/36th_Operations_Group
5th Signal Squadron - Aircraft In action 1205 - P-40 Warhawk in Action
6th Histoire & Collections - Avions et Pilotes 3 - Curtiss P-40 & P-36 1939-1945