During the 1960s, the YF-12 underwent flight evaluations by the USAF, but funding to put it into operational use was not forthcoming partly due to the pressing demands of the Vietnam War and other military priorities. It set and held speed and altitude world records of over and over (later surpassed by the closely-related SR-71 Blackbird), and is the world's largest, heaviest and fastest manned interceptor to date. Following its retirement by the USAF, it served as a research aircraft for NASA for a time, which used it to develop several significant improvements in control for future supersonic aircraft.