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