The original Curtiss XP-40, ordered July 1937, was converted from the 10th P-36A by replacing the radial engine with a new Allison V-1710-19 engine. It flew for the first time in October 1938. This new liquid-cooled engine fighter had a radiator mounted under the rear fuselage but the prototype XP-40 was later modified and the radiator was moved forward under the engine.