Preceded by the AP-7, the AP-4 possessed a superficial resemblance to the AP-2 and was intended as a high-altitude interceptor. The first Seversky fighter to feature flush-riveted skinning, the AP-4 was powered by a 1,200hp Pratt & Whitney R-1830-SC2G equipped with a belly-mounted turbo-supercharger. As a company- funded development, the AP-4 was intended to participate in a USAAC competition scheduled for 25 January 1939 and was expected to demonstrate the superior medium- and high-altitude performance obtainable with a turbo-supercharged R-1830. The same engine, fitted with a mechanical supercharger, had been specified by the USAAC for the XP-41. The AP-4 was evaluated at Wright Field during February- March 1939, demonstrating exceptional climb and altitude performance. Early 1939, it was fitted with a close-fitting, high-inlet-velocity engine cowling matched with an oversize propeller spinner as a continuation of a drag reduction programme supervised by the NACA and initiated earlier with the AP-1. The AP-4 was subsequently fitted with a modified engine cowling, without the spinner, but, on 22 March 1939, caught fire in the air, the pilot bailing out. On 12 May 1939, a contract was awarded for 13 service evaluation models under the designation YP-43.