Republic XF-12 Rainbow

The Republic XF-12 Rainbow was an American four-engine, all-metal prototype aircraft designed by Republic Aviation in the 1940s for aerial reconnaissance with the United States Army Air Forces (USAAF). Designed to maximize aerodynamic efficiency, the XF-12 was referred to as "flying on all fours" meaning: four radial engines, 400 mph (640 km/h) cruise, 4,000 mi (6,400 km) range, at 40,000 ft (12,000 m). The aircraft was redesignated as the XR-12 when the USAAF became the United States Air Force in 1947. Although the XF-12 was highly innovative, it was projected to be costly to produce and operate, and rapidly advancing technology—specifically the introduction of the jet engine—threatened to render it obsolete quickly. The prototypes were not completed until after the end of World War II, and postwar budget cuts prompted the cancellation of the production contract in favor of less expensive aircraft based on strategic bombers. Both prototypes were only used briefly for testing and were destroyed, one in a crash and the other as a practice target. A proposed airliner variant, the RC-2, was deemed uneconomical and canceled before being built.

Summary from Wikipedia, photo via Wikimedia Commons, licensed under CC BY-SA.