The SR-71 Blackbird is an awesome plane. But did you know that it could have been even more awesome than it was? The Air Force was planning to make a fighter version of the plane. Aviation historian Joe Baugher notes that as the early iterations of the SR-71 were being designed, there was a need for an interceptor. The F-108 program had been cancelled due to its high cost. However, there was still a desire for a high-speed, long-range interceptor.
The A-12 OXCART being developed for the CIA was seen as a likely basis for a fighter. Lockheed's Skunk Works team soon figured out how to add a powerful radar, the AN/ASG-18, capable of detecting targets from as far off as 500 miles in some cases, and four AIM-47 missiles.
The resulting plane was designated the YF-12, and three prototypes were built.
Designation-Systems.net notes that the AIM-47 had a range of over 100 miles and a speed of Mach 4. While a 250-ton W42 warhead never materialized for this missile, it did get a 100-pound high-explosive warhead.
News of the the YF-12's development was leaked in an effort to distract the public from the work being done to make a reconnaissance plane. But the plane – awesome as it was – would not ever see service due to the development of reliable inter-continental ballistic missiles.
The YF-12 would get a production order for 93 airframes from the Air Force. However, then-Secretary of Defense Robert McNamara refused to release the funds, and the project ended up being halted at the three prototypes. Two were handed over to NASA for research flights. One of those crashed after a fuel line caught fire in 1971, with the crew ejecting from the stricken plane.
The YF-12 will remain one of the biggest "could have been" planes in history. The jet still has a legacy – partially in the SR-71 Blackbird, but also in the form of the AWG-9 radar and AIM-54 Phoenix missiles used by the F-14 Tomcat. Even though this plane never got a chance to serve, it still did a lot for America's military aviation development.