During the 1950s. American aircraft designers emphasized configurations that flew increasingly high and fast, a trend that continued for nearly two decades. Then, during rhe 1970s, efficiency, noise reduction, and fuel economy also became important considerations, in part because military analysts no longer deemed speed and altitude the paramount capabilities necessary to ensure national security. Among the aircraft designs that transitioned from paper to hardware during the high-speed era, the Lockheed Blackbirds hold a unique place. The A-12, YF-12A, M-21. D-21, and SR-71 variants outperformed all other jet airplanes in terms of altitude and speed. To this day, they remain the only production aircraft capable of sustained cruise in excess of Mach 3. Developed in utmost secrecy, they eventually became some of the world's most famous aircraft