In early 1937, the USAAC expressed an interest in seeing how much the performance the P-36 could be improved if its radial engine were replaced by the new turbosupercharged Allison V-1710 liquid-cooled engine. On February 16, the USAAC placed an order with Curtiss for a single P-36 airframe to be powered by this new engine.
In response to this USAAC request, Curtiss's chief designer Donovan Berlin fitted a 1150 hp Allison V-1710-11 turbosupercharged engine to the original Model 75 prototype airframe. He positioned the three Prestone cooling radiators immediately behind the engine. In order to balance the aircraft and to make room for the radiators, the pilot's cockpit was moved quite far aft. Except for the cockpit relocation and the V-12 liquid-cooled engine, the XP-37 was otherwise identical to the P-36.
The modified Model 75 prototype was redesignated Model 75I by Curtiss and was delivered to the Army as a new airframe. It was designated XP-37 with Army serial number 37-375. The XP-37 flew for the first time in April 1937 and was delivered to the army in June.
The XP-37 attained a maximum speed of 340 mph at 20,000 feet and a service ceiling of 35,000 feet. An altitude of 20,000 feet could be reached in 7.1 minutes. Gross weight was 6350 lbs. The aircraft was equipped with what was the standard USAAC armament of the time--one 0.30-in and one 0.50-in machine gun mounted in the fuselage and synchronized to fire through the propeller arc.
Almost from the first, the XP-37 aircraft ran into trouble. The supercharger was extremely unreliable, and the performance of the aircraft fell short of expectations. In addition, the positioning of the cockpit that far aft on the fuselage resulted in extremely poor visibility, especially during takeoffs and landings. >p> The XP-37 was retired to an Army mechanics' school in August 1941 with a total of only 152 hours of flying time.
Although the new engine/supercharger combination was quite troublesome in the XP-37, the Army was nevertheless impressed by the potential of the design, and on December 11, 1937 they ordered 13 service test YP-37s. Serials were 38-472/484. These used Allison V-1710-21 engines fitted with improved B-2 superchargers, revised nose contours, a 25-inch increase in fuselage length aft of the cockpit, and most of the aerodynamic improvements worked out on the XP-37. The first one of these flew in June of 1939. However, the YP-37s continued to suffer with the same supercharger problems of the X-model and did not live up to their potential. All but one of the YP-37s were out of service or retired to mechanics' schools by early 1942. The highest-time aircraft had only 212 hours. The last active example (38-474) was transferred to the National Advisory Committee for Aeronautics (NACA) for research in August 1942. It survived until January 1946.
The YP-37 was powered by a 1000 hp Allison V-1710-21. Wing span was 37 feet 3 1/2 inches, length was 32 feet 11 1/2 inches, and wing area was 236 square feet. Weights were 5592 lbs empty, 6700 lbs gross. Maximum speed was 340 mph at 10,000 feet. An altitude of 20,000 feet could be attained in 8 min 30 sec. Service ceiling was 34,000 feet.
In the meantime, the USAAC had already held a competition for a new fighter in January 1939, and had chosen another Berlin design, the Model 75P which was also derived from the P-36. This was eventually to emerge as the famous P-40. All further work on the P-37 was abandoned.