In spite of the loss of the two prototypes, the USAAF was confident that there was no intrinsic flaw in the basic design, and the work on the Kingcobra project continued unabated.
The XP-63A (company Model 24, USAAF serial 42-78015) had originally been ordered as a testbed for the proposed P-63B, which was to be powered by the Packard Merlin V-1650-5. However, all available Merlin engines were urgently needed for the North American P-51 Mustang, and the aircraft was instead completed with a V-1710-93 (E-11) engine and used to support the continuing Kingcobra development program.
The XP-63A flew for the first time on April 26, 1943. The XP-63A differed from the lost XP-63s in having a 1325 hp Allison V-1710-93 engine in place of the -47. The -93 engine had a war emergency rating of 1500 hp at sea level, and this prototype was one of the fastest Kingcobras built, attaining 421 mph at 24,100 feet. External differences included the fitting of wing racks for underwing stores, a redesigned dorsal air scoop, and 12 rather than 6 engine exhaust stubs per side. The rear canopy was faired over and test equipment was installed behind the pilot. The armament was increased by adding two underwing 0.50-inch machine guns.