The YF-12 "Blackbird" was an experimental fighter-interceptor version of the Lockheed A-12 aircraft. In Air Force flight tests on May 1, 1965, the YF-12 set a speed record of 2,070.101 mph and an altitude record of 80,258 feet. First publicly displayed at Edwards Air Force base in 1964, the YF-12 was never adopted by the military as an operational aircraft. It was, however, a precursor to the SR-71 Blackbird reconnaissance plane.

Two YF-12 aircraft were flown in a joint Air Force-NASA research program at the NASA Flight Research Center (now the NASA Dryden Flight Research Center) between 1969 and 1979, although the second plane, piloted primarily by the Air Force, was lost to an inflight fire in 1971. The two YF-12 aircraft bore the serial numbers 60-6935 and 60-6936.

The YF-12 allowed NASA researchers at all four of the agency aeronautical centers (Langley, Lewis, and Ames as well as the Flight Research Center) to study the thermal, structural, and aerodynamic effects of sustained, high-altitude, Mach 3 flight.

Painted flat-black, the YF-12 was fabricated primarily from titanium alloy, which enabled it to withstand skin temperatures of over 500 degrees Fahrenheit.

Work on the YF-12 began in secret in the late 1950's at the Lockheed Advanced Development Projects office, better as known the "Skunk Works," in Burbank, California. Flight data remained classified long after President Lyndon Johnson announced the plane's existence on Feb. 29, 1964. After the announcement, the plane received the Air Force designation YF-12A.

Although it yielded large amounts of research data, the YF-12 program was terminated in the late 1970's when the NASA research agenda had shifted from speed to efficiency. During its nine-year life, the YF-12 research program logged 297 flights and approximately 450 flight hours (including hours in an SR-71A designated YF-12C, on which there is more information below).

The YF-12 carried a crew of two -- a pilot and a flight engineer. Research pilots Fitzhugh Fulton and Donald Mallick flew NASA YF-12 flights at the NASA Flight Research Center (redesignated the Dryden Flight Research Center in 1976) from 1970 to 1979, with several other pilots performing familiarization flights. The flight engineers on the YF-12 crews were Victor Horton and Ray Young. Air Force crews also conducted testing.

Under its research agreement with NASA, the Air Force provided the agency with two YF-12A aircraft in 1969. On June 24, 1971, one of the planes (serial no. 60-6936) experienced an inflight fuel line failure and engine fire. Unable to save the smoking aircraft, Air Force pilot Lt. Col. Ronald Layton and systems engineer Major Billy A. Curtis ejected and were not injured, but the YF-12A was lost in a fiery explosion in the desert. About this time, Dryden acquired a so-called YF-12C, which was in fact a then-secret SR-71A (serial no. 61-7951) given the NASA tail no. 60-6937. The reason for this bit for subterfuge lay in the fact that NASA, while flying the YF-12A interceptor version of the aircraft, was not allowed to possess the strategic reconnaissance version for some time. The bogus tail number actually belonged to a Lockheed A-12 (serial no. 60-6937), but the existence of the A-12 remained classified until 1982. The tail number 06937 was selected because it followed in the sequence of tail numbers assigned to the three existing YF-12A aircraft: 06934, 06935, and 06936. The SR-71 differed from the YF-12A in that the YF-12A had a round nose while the SR-71 had its chine carried forward to the nose of the airplane. There were other differences in internal and external configuration, but the two aircraft shared common inlet designs, structural concepts, and subsystems.

The YF-12 project provided a wealth of information on systems integration, Mach 3 handling and performance, aerodynamics, propulsion, controls, structures, and subsystems.

The program was ordered terminated in 1977, but NASA used some residual funding to keep the project alive into 1979. Plane 06935 made its last NASA flight on October 31, 1979.

Only one YF-12 remains in existence. It is displayed at the US Air Force Museum at Wright-Patterson Air Force Base in Dayton, Ohio.

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