In 1955, USAF launched the program of LRIX (Long-Range Interceptor Experiment), that the fighter was to be capable of March 3 performance and could destroy attacking Soviet bombers over the poles before they could get near U.S. territory.
It was also to be the escort fighter for the XB-70 bomber. As the XB-70 was
developed and built by North American Aviation, the company was also issued a contract for two prototypes which designated F-108A Rapier. The F-108A was designed as a large delta-winged aircraft, powered by a pair of after burning turbojets. A mockup was inspected in January 1959 with planned first flight in 1961. However, by mid 1959, the threat from Soviet Union was perceived to be its intercontinental ballistic missiles instead of its force of long-range bombers. Against the ballistic missiles, F-108 would be completely useless.
The program was cancelled on September 1959.