Supermarine Spitfire. At Royal Air Force's service. Part seven. The Battle of Malta, part one.

 

During January 1942 the first Westland-built Spitfire flew for the first time, pushing the number of manufactured-Spitfires to more than 3300 at the end of the year. By June 1942 there were 59 Spitfire Mk.V squadrons based in Great Britain, and seven additional more in the Mediterranean. In August there were no less than 42 Spitfire Mk. V squadrons to cover the Dieppe Raid.
The Mk. V was the most numerous of the Spitfire variants and eventually equipped more than 100 Squadrons of the Royal Air Force (RAF). It saw action on every battle front were the RAF or Allied' air forces were present.
By 1942 the threat of a land invasion of England fell considerably and the main focus of the war turned to the USSR and the Mediterranean. It was in Malta, where by the spring 1942 the defence of that island was becoming critical. Only the Spitfires could challenge the German fighters on equal terms, so the first tropicalized Spitfires Mk. Vs were delivered via the aircraft carriers such as USS Wasp or HMS Eagle. The first Spitfire flight in the Mediterranean took place earlier, on 7th March 1942 when 15 Mk. Vs took off from HMS Eagle aircraft carrier, off the coast of Algeria to land on Malta. 
Provided with a large and peculiar under-nose fairing over the Vokes Multi-vee filter for the carburetor air intake, the Mk. Vc was also equipped with enlarged oil and radiator intakes to optimize cooling in tropical and desert climates. The improve its range, three sizes of fuselage slipper tanks -30 gal. for short range, 90 gal. for long range and 170 gal. for ferrying could be attached in the belly. 
During the spring months of 1942 around 275 Mk. Vb and Mk. Vcs were delivered to Malta. In order to make possible the Spitfire to take off from an aircraft carrier wooden wedges were used to allow the Spitfires to leave the carrier with partial take-off flap settings, so, when the aircraft gained enough altitude, the pilot opened the flaps completely, the wedges fell out and then the flaps could be closed. 
On 20th April 1942 the code-named "Operation Calendar" took place, when a total of forty-seven Spitfires and pilots from Nos. 601 and 603 Squadrons flew from USS Wasp to Malta. In "Operation Bowery", on 9th May, additional 50 Spitfires flew from USS Wasp and 14 from HMS Eagle, sixty of them managed to land safely on Malta, with one Spitfire with a defective long range fuel tank landing back on USS Wasp, despite not having a tailhook. In "Operation Style", on 3rd June, further 32 Spitfires flew to Malta from HMS Eagle, although they were intercepted while on route and four were shot down. 

Sources:
1st Signal Squadron - Aircraft In action 39 - Supermarine Spitfire in Action
2nd https://en.wikipedia.org/wiki/Supermarine_Spitfire_operational_history#Mediterranean_service

Supermarine Spitfire. At Royal Air Force's service. Part six. The Spitfire Mk.V

 

(This post is a direct continuation from our previous post) 
As we wrote, a new complete redesign was not desirable for Spitfire because it disrupted the production lines, so in February 1941 No. 92 (East India) Squadron of the Royal Air Force (RAF) became the first one to receive examples of the Mk. V which was, at first, considered as an interim fighter. This was followed by No. 91 (Nigeria) Squadron in March. Those machines were part of the initial production order for 1.000 Mk. Vs.
The Mk. V was an Mk. I/II airframe strengthened with longerons to fit a Merlin 45 engine rated at 1470 hp and by June production of the Mk. Va terminated in favour of the Mk. Vb which featured two 20mm cannons plus four 0.303 in machine guns in the wings. 
Early examples of the Mk. Vb had the same span of the Mk. I/II, although, in order to improve performance at low altitudes, clipped wings were introduced later. The number of small improvements that the Mk. Vb got was very high throughout its manufacture lines and at service levels. One of the most significant was a jettisonable cockpit canopy with more bulbous edges, replacement of metal-covered ailerons and the replacement of flame-damping exhaust stubs.
By September 1941 a total of twenty-seven RAF squadron were operating the Mk. Vb, while very operated the Va. In December those numbers rose to forty-six. 

The Spitfire Mk. III was a prototype variant, of which a single example was completed, N3297. It was the first significant redesign of the type and differed from previous marks by having clipped wings, a strengthened fuselage, a retractable tailwheel, an internal bullet-proof windshield and 88 lbs of armour plating. Two additional inches more forward rake was added to the strengthened undercarriage and the wheels were fully enclosed by doors when retracted.
Priority for the Hawker Hurricane Mk. II was given, so the Spitfire Mk. III wasn't put into production and was employed as a test-bed for different engines (among them the Merlin 60 and 61 - the same one that would eventually power the Mk. IX) and armaments until September 1944. 

The Spitfire F.IV was the prototype for the 'F' sub-mark which was powered by the Roll-Royce Griffon engine. Its inception dates back to 4th December 1939 when Supermarine design staff printed a brochure where they proposed the idea of converting the Spitfire to the Rolls-Royce Griffon engine, expecting many performance improvements. However, the problematic development of the Griffon engine meant that the prototype of this machine would not be ready until late 1941.
The Griffon IIB which the Mk. IV was fitted with was a single-stage supercharged engine of 1735 hp. This required stronger longerons to cope with the weight of the engine, as well as bigger radiator and oil cooler, although it kept the asymmetric under-wing radiator layout of the single stage Merlin engine. The lower thrust line and larger capacity of the Griffon engine required a complete redesign of the engine cowling, with more prominent blisters over the cylinder heads and a third tear-drop shaped blister on the upper forward cowling to clear the magneto, and a deeper curve down to the spinner, which was much longer than previous marks. A four-bladed Rotol propeller was employed. This propeller catch a lot of pilots out because it rotated in the opposite direction to that of the Merlin: to the left from the pilot's perspective, instead of to the right, meaning that the sliptstream swung the Spitfire to the right on the ground, requiring the rudder to be turned to the left during take-off.
The Mk. IV DP845 flew for the first time on 27th November 1941 and was equipped with the full-span C type wing. It featured a small tail unit and retractable tailwheel. It had also external bracket hinges under the wings, denoting the installation of braking flaps, which were soon removed and the F.IV was soon also fitted with a mock-up six-cannon armament, three per wing. The aircraft was renamed to 'Mk. XX' to prevent confusion with a PR. Type and, then it became the Mk. XII. 

Sources:
1st Signal Squadron - Aircraft In action 39 - Supermarine Spitfire in Action
2nd https://en.wikipedia.org/wiki/Supermarine_Spitfire_(Griffon-powered_variants)
3rd https://en.wikipedia.org/wiki/Supermarine_Spitfire_(early_Merlin-powered_variants)

Supermarine Spitfire. At Royal Air Force's service. Part five. After the Battle of Britain.

 

The Spitfire Mk. II was, externally, identical to the Mk. I. It incorporated all the improvements made to the Mk. I. It was powered by the Merlin XII engine rated at 1.175 hp, which employed 100 octane fuel, instead of the 87 octanes one. The Mk. II was manufactured at Castle Bromwich, Birmingham with the first airframe being completed in June 1940 and the first 750 Mk. IIs were equipped with the "a" type wing, while the remaining 170 had the "b" type wing. Cockpit protection was improved and self-sealing fuel tanks were also introduced, albeit many Mk. Is were already retrofitted with them.
Some few experiments were carried out on Mk. IIs, specially aimed to increase Spitfire's operational range of 395 miles (636 km). This was done because the Spitfire was designed as a home-based defence fighter with the smallest possible airframe, so it had little space for extra fuel tanks. Therefore, various external fuel tank configurations were tested, including a 40 gal (151 L) tank faired into the wing leading edge. The Spitfire Mk. II (LR), as it was denominated (LR stands for 'Long Range'), was employed by Nos. 66, 118 and 152 squadrons for a few operations. However, the fact that the tanks couldn't be jettisoned and had an adverse effect on handling the aircraft, caused them to be withdrawn in favour of the fuselage 'slipper' tank.
The Spitfire Mk. II was also the first Spitfire to carry bombs, although of the non-lethal smoke ones. When the last fifty-two examples became Mk. IICs (the letter "C" meant a change in role instead of a wing configuration). These Mk. IICs were officially known as 'Sea Rescue Type E' and carried a small dinghy boat and food supplied to be parachute-dropped. They were employed by five squadrons and redesignated as ASR (Air Sea Rescue) Mk. IIs in late 1942.

After the end of the Battle of Britain, Royal Air Force's (RAF) Fighter Command put to rest many of the squadrons that carried the bulk of the fight and prepared for a campaign of offensive sweeps across the Channel. During this period of time, the Hawker Hurricane was gradually phased out and, in many cases, replaced by the Spitfire. For those fighter sweeps, day fighter squadrons developed a framework of code-names, the most important of which were "Rhubabr" - small scale attacks by fighters of fighter-bombers; "Circus" - heavy escort to light bombers which acted as bait to attract enemy fighters into the battle; "Ramrod" - just like Circus but with the main objective of destroying the bombers' target; "Roadstead" - an attack by bombers with fighter escort on shipping targets; "Rodeo" - a straightforward fighter sweep over enemy territory and finally "Ranger" - a freelance penetration of enemy airspace in squadron or wing strength with the objective of wearing down the defences. 
Such operations were risky for both men and machines and, although the Spitfire squadrons were able to hold against the German Messerschmitt Bf.109E and F variants, the advent of the Focke-Wulf Fw.190 in autumn 1941 and the many ground anti-air fire of all calibres, changed the odds. In fact, many Spitfires were lost to ground fire of all calibre, ranging from rifle rounds to purpose-built flak guns. Among those victims were 'Paddy' Finucane and Bob Stanford-Tuck, aces of the Battle of Britain.
During early 1941, the Spitfire was equipping most operational fighter squadrons and every effort was made behind the scenes, to ensure that it remained at least equal to its Luftwaffe adversaries. Nonetheless, it was undesirable to disrupt production of the Spitfire by introducing radical changes, so a new mark, Mark V was put underway. (To be continued)

Sources:
1st Signal Squadron - Aircraft In action 1039 - Supermarine Spitfire in Action
2nd https://en.wikipedia.org/wiki/Supermarine_Spitfire_operational_history#European_offensive_1941–1943

Supermarine Spitfire. At Royal Air Force's service. Part four. Battle of Britain

 

The Battle of Britain constituted the first major test for the Supermarine Spitfire, whose performance was often compared to that of the Hawker Hurricane, the main fighter of Royal Air Force's (RAF) Fighter Command. 
The Hurricane had thicker wings and its eight 0,303 in machine guns were easier to install and set-up than the Spitfire thanks to having a thicker wing and being closer the one from the other. Some veteran pilots from the Battle of France requested the machine guns to be harmonised, so their combined fire bursts could concentrate in 250 yards (228.6 m) ahead instead of the official 400 yards (365.76 m). This made the Hurricane a better gun platform than the Spitfire.
The Hurricane shot down more enemy aircraft than the Spitfire, thanks to its higher proportion in the skies. In fact, during the Battle of Britain, seven out of ten German aircraft were shot down by Hurricane pilots. However, losses were also higher among the many Hurricane squadrons. According to post-war studies, the kill-ratio of the Spitfire was, however, slightly better than that of the Hurricane.
Most of the Spitfires were armed with a total of eight 0,303 in machine guns, four per wing. It was very common for Luftwaffe's pilots to return home with 0,303 in bullet holes in their fuselages, without having sustained critical damage as their main fighter, the Messerschmitt Bf.109, had received armour plating in critical areas and self-sealing fuel tanks became common in their bombers. 
Those improvements made to German aircraft, required some improvement in armament, so many Spitfires Mk. I of No. 19 Squadron (the first unit to employ them) were fitted with two 20mm Hispano-Suiza cannons. This arrangement, however, proved to be unreliable, as the cannons often failed to fire, or they got jammed very easily. After complaints, those cannons were replaced with conventional machine guns in September 1940. This arrangement was, however, the forerunner of the Mk. Ib variant, which would become standard shortly later and would name, retrospectively, the not-cannon-armed Mk. I variant as the "Mk. Ia". 

The Merlin III engine was rated, theoretically, at 1.030 hp, however it required 100 octane fuel from the United States, which became available only from early 1940. This meant that the emergency boost could last only for five minutes, The extra boost wasn't dangerous as long as the pilot's notes were acknowledged. Anyway, as a precaution, if the boost had been used, the pilot had to report it upon landing, so it could be noted in the engine's log book. This extra boost became standard and more optimized on subsequent variants of the Spitfire. 
Between 1st August and 31st October 1940 a total of 208 Spitfires were lost in combat, seven of them destroyed on the ground and 42 in accidents.

The Speed Spitfire, as we have seen in a previous post, was a speed record variant which was pushed into RAF's service and fitted with photographic cameras, thanks to its high speed. It was one of the fastest aircraft in the world, however it suffered from reduced fuel capacity, so it was, eventually, used as a high-speed hack for the personal use of Air Commodore Boothman. 
It was eventually struck off charge on 14th June 1946 and scrapped. 

Sources:
1st https://en.wikipedia.org/wiki/Supermarine_Spitfire_operational_history#Battle_of_Britain
2nd https://en.wikipedia.org/wiki/Aircraft_of_the_Battle_of_Britain
3rd https://en.wikipedia.org/wiki/Supermarine_Speed_Spitfire

Supermarine Spitfire. At Royal Air Force's service. Part three. The Spitfire in the Battle of France.

 

The Spitfire Mk. III, pictured below, was an attempt to improve the basic Spitfire design. It was powered by a Rolls-Royce RM 2SM, which was later known as the Merlin XX and yielded 1,390 hp of power, thanks to its two-speed supercharger and reduced wingspan. In order to increase ground stability, the undercarriage was raked forward, and had flaps to completely enclose the wheels when retracted. The tailwheel was fully retractable too. 
The windscreen was remodelled, with a built-in internal bulletproof glass panel, and was armed with the "c-type" wing, which could host four 20 mm cannons, eight 0.303 in machine guns or two cannons and four machine guns. Maximum speed was increased to 385 mph (620 km/h).
The first prototype, registered as N3297 flew for the first time on 16th March 1940 and was thoroughly tested and, although the Mk. III was ordered for mass production, it was soon cancelled as the Merlin XX engine was in short supply and was needed for the Hawker Hurricane Mk. II, which was the workhorse of the Royal Air Force. In the meantime Rolls-Royce developed the Merlin 45, a similar engine in terms of performance, which could be fitted in the fuselage of a Spitfire Mk. I or a Mk. II. The Mk. III was abandoned in favour of the Mk. V, although the improvements first seen in the Mk. III were incorporated in later marks of the Spitfire. The airframe of the Mk. III was used to test the Merlin 61 engine, which later powered the marks VII, VIII and IX, making the Mk. III the true ancestor of those marks.

Some other Spitfires were also altered, one of those was R6722,  which, when the Germans invaded Norway in April 1940, the RAF showed interest in the concept of floatplane fighters, which could be useful in those areas were airfields were not immediately available, so R6722 was taken from the Woolston factory, to be fitted with floats taken from a Blackburn Roc naval fighter. The conversion was made by No. 12 Maintenance Unit at RAF Kirkbride in Cumbria and later, tank tests were carried out at Farnborough, employing a scale model, finding that the concept was solid even if some changes were to be made, such as enlarging vertical tail surfaces to counter-balance the side area of the floats. The "Narvik Nightmare", that's how this Spitfire was commonly known, was reverted back to a regular Mk. I when the Battle of Norway ended and, eventually, it was upgraded to a Mk. V. 

The Spitfire was first used at an operational level over western Europe on 13th May 1940, during the German invasion of the Netherlands. Since three days earlier, German paratroopers were pinned down by the Dutch Army in the Battle of Hague, so the Royal Air Force sent No. 66 Squadron, equipped with Spitfires to escort No. 264 Squadron, equipped with Boulton Paul Defiants, to provide ground support for the Dutch army. The Spitfires encountered German Junkers Ju.87 Stukas belonging to IV (St.)./Lehrgeschwader 1 and shot down four of them. However, they were soon intercepted by Bf.109 from 5./Jagdgeschwader 26 that shot down five Defiants and one Spitfire, for the loss of just one Bf.109.On 23rd May 1940 other German Bf.109s were shot down by Spitfires of No. 54 Squadron over Calais Marck airfield, on the northern shore of France.
During the Battle of France a total of 67 Spitfires were lost on French soil, most of them during attempts to prevent the Luftwaffe from bombing the evacuation beaches at Dunkirk. During this period, every Spitfire unit was based in Great Britain, as Air Vice Marshal Hugh Dowding ordered, however, from late 1939 some Spitfire PR.I, specially modified for photo reconnaissance duties, belonging to No. 2 Camouflage Unit, were operating from Seclin, northern France, close to the border with Belgium, performing photo-intelligence gathering of German defences and cities. 

The Battle of Britain began on 10th July 1940 and became the first major test for both the Spitfire and the RAF Fighter Command. During this time, very important lessons were learned about Spitfire's capabilities and setbacks, but that's for another post.

Sources:
1st https://en.wikipedia.org/wiki/Supermarine_Spitfire_operational_history
2nd https://en.wikipedia.org/wiki/Supermarine_Spitfire_(early_Merlin-powered_variants)
3rd https://www.historyofwar.org/articles/weapons_spitfire_mkIII.html
4th https://en.wikipedia.org/wiki/Supermarine_Spitfire_(late_Merlin-powered_variants)
5th https://www.solentsky.org/post/secret-files-the-spitfire-floatplanes
6th https://allspitfirepilots.org/aircraft/R6722

Supermarine Spitfire. At Royal Air Force's service. Part two. Prototypes and early Spitfires.

 

After the disappointment of the Type 224, Mitchell and his design staff, began to draw cleaner designs using the experience with the Schneider Trophy seaplanes as a departing point. This, eventually led to the Type 300, which featured retractable undercarriage and a reduced wingspan. This original design was submitted to the Air Ministry but it was rejected, so it had to undergo a series of changes such as an enclosed cockpit, oxygen-breathing gear, smaller and slimer wings, and it was to be powered by the, back then, cutting edge Rolls-Royce PV XII V-12 engine, which was later commonly known as the Rolls-Royce Merlin. 
In November 1934, Mitchell was given permission from Vickers-Armstrong (Supermarine's parent company) to develop and reife the Type 300 and on 1st December he was given £ 10,000 from the Air Ministry for the construction of the Type 300, under contract AM 361140/34. 
On 5th March 1936 the first prototype, registered as K5054, took-off for the first time at Eastleigh Aerodrome, Hampshire. At the controls was Cpt. Joseph "Mutt" Summers, chief test pilot for Vickers, who praised the design. 
The initial prototype was later fitted with a new propeller and it was flown again on 10th March 1936, where the undercarriage was retracted for the first time and, after a fourth flight, a new engine was fitted. After overcoming many small failures and improving the design, it was flown again to RAF Martlesham Heath where it was handed to Squadron Leader Anderson of the Aeroplane & Armament Experimental Establishment (A&AEE) to be delivered to the Royal Air Force (RAF). Soon after the Air Ministry placed a production order for 310 Spitfires.
Although it was presented to the public on 27th June 1936, numerous problems hindered the mass production of the Spitfire and the first production aircraft wasn't completed until mid-1938 and it was delivered to No. 19 Squadron at RAF Duxford on 4th August 1938 and during the next weeks No. 66 Squadron also received the type, albeit at a slower pace. The next unit to receive the Spitfire was No. 41 Squadron at RAF Catterick and the first public sight of the Spitfire in RAF's colours and markings took place on Empire Air Day, on 20th May 1939. Later, more squadrons were gradually equipped with the type.
When World War 2 began, there were a total of 306 Spitfires in service with the RAF, seventy-one of them in reserve and 2.000 under manufacture with 36 written off due to accidents.
On 6th September 1939, shortly after the outbreak of the war, a friendly-fire incident known as the Battle of Barking Creek took place where two Hawker Hurricanes of 56 Squadron were shot down by Spitfires of 74 Squadron over the river Medway, Kent. One of the victims of this incident, P/O Montague Leslie Hulton-Harrop, was the first British pilot fatality of the World War II. This incident placed the manufacture of IFF (Identification Friend or Foe) equipment as a top priority.
On 16th October 1939 the Spitfire clashed against the German Luftwaffe for the first time when three aircraft from 602 and 603 Squadrons intercepted three Junkers Ju.88 belonging to 1./KG.30 over Rosyth (Fife, Scotland) trying to attack the cruiser HMS Southampton and HMS Edinburgh on the Firth of Forth. Two of the Ju.88 were shot down and another one was heavily damaged.

The PR (Photo-Reconnaissance) variants of the Spitfire, was a sub-variant dedicated for photo-reconnaissance missions. Before the war, military aerial doctrine dictated that the best option for long-range reconnaissance duties was to employ converted bombers, as they had defensive armament, however, it was soon discovered that Bristol Blenheims and Westland Lysanders were easy targets for German fighters so, following a memorandum made back in August 1939 by Flying Officer Maurice Longbottom, two Spitfires, N3069 and N3071 were released from RAF Fighter Command and sent to the Heston flight, a secret reconnaissance unit. 
Those two Spitfires were stripped from their armament and radio gear, two F24 ventral cameras were installed, heating equipment was added to prevent photographic cameras from freezing and the lenses from frosting. These Spitfires were later known as Spitfire Mk. I PR Type A and achieved a high speed of 390 mph (627 km/h) and were employed for the first in late 1939 to perform photographic reconnaissance flights over western Germany.

The Speed Spitfire was an racer-built variant which was built in late 1938 to achieve world speed records, mainly against German aircraft such as the Messerschmitt Bf.109 V13, Heinkel He.100 V2 and the Messerschmitt M.209 V1, which eventually got the prize achieving a top speed of 755.14 km/h (469.22 mph) on 26th April 1939.
It featured many design changes to make the type more aerodynamic and lacked radio equipment and armament to make it lighter. The Speed Spitfire was eventually handed to the RAF at the beginning of World War 2 where it was suited for photographic reconnaissance, given its high top speed. 

Sources:
1st https://en.wikipedia.org/wiki/Supermarine_Spitfire
2nd https://en.wikipedia.org/wiki/Supermarine_Spitfire_operational_history
3rd https://en.wikipedia.org/wiki/Supermarine_Speed_Spitfire
4th https://en.wikipedia.org/wiki/Supermarine_Spitfire_variants:_specifications,_performance_and_armament
5th https://en.wikipedia.org/wiki/Supermarine_Spitfire_(early_Merlin-powered_variants)

Supermarine Spitfire. At Royal Air Force's service. Part one. The Type 224 Spitfire.

 

The Supermarine Spitfire was a British fighter of the World War II era. It is regarded as one of the best fighters of the war. It has its origins in an Air Ministry Specification; F.7/30 from October 1931 which sought an all-metal day and night fighter armed with four machine guns, a good cockpit visibility, decent high maximum speed and good rate of climb to replace the ageing Gloster Gauntlet. 
The Air Ministry favoured the usage of the Rolls-Royce Goshawk engine, although it wasn't exclusive, so any kind of engine could be employed. 
Bristol, Hawker, Westland, Blackburn and Supermarine submitted proposals, but only three were selected for prototype production: Supermarine Type 224, Blackburn F.3 and Westland F.7/30, with designs from both Bristol and Hawker being rejected. 
Given Supermarine's success in the Schneider Trophy contests, R.J. Mitchell (Supermarine's chief design), was confident in designing a high-speed fighter, so four months after receiving the specification, a monoplane design was submitted, the Type 224.

The Type 224 featured an inverted gull-wing, had a fixed landing gear encased in large fairings and the fuselage was of monocoque construction. It was armed with four Vickers Mk. IV machine guns, two in either side of the open cockpit and the whole fighter was powered by a Rolls-Royce Goshawk II piston engine rated at 600 hp of power.
After wind tunnel tests, Supermarine submitted theoretical performance data to the Air Ministry , suggesting that the aircraft would enjoy good enough performance, so the Air Ministry granted Supermarine a prototype contract. 
On 19th February 1934, the Type 224 took off for the first time, at the hands of Joseph "Mutt" Summers, chief test pilot of Supermarine. Its performance was, however, disappointing, with lateral stability problems caused by the wing design, so a series of redesigns were carried out to the model, among them, an enlarged fin. 
Mitchell proposed a series of improvements to the Type 224, such a new wing, tailplane and a new engine, which would increase performance, but the Air Ministry decided in favour of a new fighter aircraft, instead of a modification of the Type 224. In January 1935 further production was cancelled. 
On 25th May 1937 the only Type 224 was transferred to the Aeroplane and Armament Experimental Establishment at RAF Martlesham Heath, Suffolk to be used during the summer for target practice at Orford Ness, Suffolk.

Although the Type 224 was a failure, Mitchell pushed on and kept designing a fighter, with a complete redesign feature a new straight wing and a thinner airfoil shape. 
The new redesign quickly caught Air Ministry's attention, so in January 1935, shortly after cancelling production on the Type 224, they issued Specification F.37/34 for the sole purpose of covering a new design by Supermarine. This specification mentioned purposely Supermarines specification 425a drawing and, eventually, led to the design of the Supermarine Spitfire.
Supermarine asked the Air Ministry to reserve the name 'Spitfire' to be reserved for the Type 224 and it was adopted, however Mitchell disliked it, so it was later dropped.

Sources:
1st https://en.wikipedia.org/wiki/Supermarine_Type_224
2nd Putnam Books - Supermarine Aircraft since 1914