On 3 December 1942, the USAAF sent out a Request for Proposals (RfP) for a diminutive piston-engined fighter. By January 1944, the Air Technical Service Command refined the RfP and, in January 1945, the specifications were further revised in MX-472 to specify a jet-powered aircraft. Although a number of aerospace companies studied the feasibility of such aircraft, McDonnell was the only company to submit a proposal to the original 1942 request and later revised requirements. The company's Model 27 proposal was completely reworked to meet the new specifications.