designated the A-12, the Blackbird was designed at the
request of the CIA for an mach 3+ over-flight reconnaissance
aircraft to replace the U-2.
The A-12's performance led to the development of fighter and bomber
variants designated as the F-12 and B-71. Three
prototype YF-12A fighters were built while the reconnaissance
bomber version was dropped.
In the meantime, a more capable reconnaissance version was
developed which kept the bomber version's sequence number but used the
one-of-a-kind SR designation (strategic reconnaissance).
Another version (the D-21) was built to
carry the D-21 reconnaissance drone.
The first SR-71 flew in 1964 and entered service in 1966. The
USAF retired its SR-71 fleet in 1990 due to high operating costs.
The Blackbird began flying missions again in 1997 and returned to
retirement in 1998.
During its operational career, the SR-71 was the world's fastest and
highest-flying operational aircraft with an absolute speed record of 2,193
miles per hour and absolute altitude record of 85,069 feet.