The XP-37 was the first attempt to match the existing P-36 (Hawk 75) fighter with a turbo-supercharged Allison liquid cooled inline engine. The P-36 used a radial engine, but all the evidence from Europe suggested that the more streamlined inline engines were going to produce better fighters. The new aircraft was given the designation XP-37 by the USAAF, and Hawk 75-I by Curtiss.
The problem faced by the designers of the XP-37 (and the later P-40) was that although the basic Allison V-1710-11 engine was similar in size to the early Rolls Royce Merlin used in the Hurricane and Spitfire, it needed a turbo-supercharge to achieve similar high altitude performance. The combination of Allison engine and turbosupercharger also took up more space than the radial engine used in the P-36. The designers of the P-36 responded by making two changes to the fuselage of the P-36. First, they added a longer nose, to contain the engine itself. Second, they moved the cockpit to the back of the fuselage, to make space for the radiators required by the liquid cooled engine.
The resulting aircraft was fast. Powered by the Allison V-1710-11 engine, which gave 1,000 hp at 20,000 feet, it became the first American fighter to reach 300 mph in level flight, reaching a top speed of 340 mph. The turbo-supercharger gave the XP-37 much better performance at altitude than the aircraft that replaced it, the P-40. However, the turbo-supercharger was not yet ready for active service.
A bigger problem was the poor visibility caused by the long nose and poor cockpit position. Thirteen service-test YP-37s were ordered, but they did nothing to improve the situation – although the cockpit was moved slightly further forward, the nose was made longer, so the poor visibility remained. The P-37 design was abandoned in favour of the P-40, which had a more traditional cockpit position, and a more reliable supercharger.