Republic XF-12 Rainbow

The Republic XF-12 Rainbow was a four-engine, all-metal prototype designed by Republic Aviation in the 1940s for long-range aerial reconnaissance with the United States Army Air Forces. It was referred to as flying on all fours: four radial engines, a 400 mph cruise, a 4,000 mile range and a 40,000 foot ceiling. It first flew on 4 February 1946 and in testing reached 45,000 feet and demonstrated a range of 4,500 miles, beating its design criteria, and it was the fastest aircraft of its day to use four reciprocating engines. Only two prototypes were built, because postwar budget cuts ended the production contract in favor of cheaper aircraft based on strategic bombers; one prototype was lost in a crash and the other became a practice target. A proposed airliner version, the RC-2, was judged uneconomical and canceled before it was built.