After the F-100 Super Sabre was in production, the design was reconfigured to meet Air Force requirements for a fighter-bomber capable of delivering a nuclear payload at Mach 2 speeds. The first versions were designated F-100B, but when the U.S. Air Force ordered prototypes, the designation changed to YF-107A (NA-212). The air intake was located behind the cockpit to permit installation of a fire control radar. The YF-107A was the last fighter North American Aviation built for the USAF, and it flew Mach 2 in its first all-out test flights during 1957. The three YF-107As provided valuable data for advanced flight research. They were followed by the F-108 (NA-257) Rapier, which entered the design stage as a Mach 3 interceptor, but was never produced.