In October 1942, the contract for two prototypes, designated "XP-75", was signed with the Fisher Body Division of GM. The design concept was to use the outer wing panels from the North American P-51 Mustang, the tail assembly from the Douglas A-24 (SBD), and the undercarriage from the Vought F4U Corsair in a general layout much as in the Bell P-39 Airacobra with the engine located with the contra-rotating propellers driven through extension shafts. At an early design stage, however, Curtiss P-40 Warhawk outer wing panels were substituted for the P-51 panels.