North American XB-70 Valkyrie was designed by the USAF Strategic Air Command in the 1950s, in response to a request for a nuclear bomber that would fly at 3 times the speed of sound (Mach 3) with its 6 engines. At this speed, it was expected that the XB-70 would be practically immune to interceptor aircraft, the only effective weapon against bomber aircraft at the time. The bomber would spend only a brief time over a particular radar station, flying out of its range before the controllers could position their fighters in a suitable location for an interception. Its high speed made the aircraft difficult to see on radar displays and its high-altitude and high-speed capabilities could not be matched by any contemporaneous Soviet interceptor or fighter aircraft. The proposed cost of the project led to the cancellation of the program, although two prototypes produced made successful flights in the 1960s. One of the prototypes crashed in an air accident in 1966, and the other prototype was placed on display at the National Museum of the US Air Force.