The SR-71, unofficially known as the "Blackbird," is a long-range, advanced, strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft. The first flight of an SR-71 took place on December 22, 1964, and the first SR-71 to enter service was delivered to the 4200th (later, 9th) Strategic Reconnaissance Wing at Beale AFB, California, in January 1966. The U.S. Air Force retired its fleet of SR-71s on January 26, 1990, because of a decreasing defense budget and high costs of operation. The USAF returned the SR-71 to the active Air Force inventory in 1995 and began flying operational missions in January 1997. Throughout its nearly 24-year career, the SR-71 remained the world's fastest and highest-flying operational aircraft. An air-to-air front view of an SR-71A strategic reconnaissance aircraft. The SR-71 is unofficially known as the "Blackbird."