Check out this NASA video about the Mach 3+ Lockheed YF-12 Blackbird interceptor, which was developed from the Central Intelligence Agency’s A-12 reconnaissance aircraft. Though ultimately it would not go into production, the three pre-production aircraft did find success as research planes for NASA.
The YF-12 was developed after the cancellation of the North American XF-108 Rapier—another Mach 3+ interceptor—using many of the same mission systems including the Hughes AN/ASG-18 pulse-doppler radar and long-range Hughes AIM-47 Falcon missiles.[youtube]http://youtu.be/Z1FLEbAbl4Q[/youtube]
Though the USAF did order 93 F-12B production jets for its now-defunct Air Defense Command, as the threat of Soviet bomber attacks on the continental United States diminished, the need for such an interceptor evaporated. The program was cancelled in early 1968 by then SecDef Robert McNamara.
Following the program’s termination, the planes were flown by NASA pilots for about 10 years or so.