The
Republic XF-12 Rainbow was a high-altitude, long-range strategic reconnaissance aircraft designed by
Republic Aviation in the late 1940s. Developed to meet a 1943 U.S. Army Air Corps requirement for a plane that could cruise at
400 mph, operate at
40,000 feet, and cover
4,000 miles (a specification nicknamed "flying on all fours"), the aircraft was intended to photograph enemy targets from beyond the reach of interceptors.
Only
two prototypes were built, with the first flight occurring on
February 4, 1946. The XF-12 became the
fastest four-engine piston aircraft ever built, reaching speeds of
470 mph at altitudes of
45,000 feet with a range of
4,500 miles. Its innovative design included an onboard darkroom for developing film in flight, allowing for immediate intelligence analysis upon landing.
Despite its superior performance, the program was
canceled shortly after World War II ended. The advent of
jet engine technology and the availability of cheaper converted bombers (like the RB-50) made the complex, piston-powered Rainbow obsolete before it could enter production. The aircraft was officially retired in
June 1952.