The fourth production P-36 (serial 38-004) became a development platform for a direct successor, designated XP-42 by the USAAC. The XP-42 was powered by a
Pratt & Whitney R-1830 "Twin Wasp" engine fitted with a longer, streamlined cowling and a large propeller spinner. These features attempted to improve the aerodynamics of the air-cooled
radial engine. When the XP-42 first flew in March 1939, it proved to be faster than the P-36. However, the P-40 was faster still and the new nose cowling caused engine cooling problems that proved to be unresolvable, despite at least 12 sets of modifications, which ultimately resulted in a shorter and less streamlined cowling closely resembling the original P-36 Hawk. The XP-42 project was canceled. However, the XP-42 prototype was retained as a test-bed and was later fitted with an all-moving tail (
stabilator), for research purposes. This aircraft was scrapped on July 15, 1947. ==Specifications (XP-42)==