The Republic XF-12 Rainbow was an American four-engine, all-metal prototype reconnaissance aircraft designed by the Republic Aviation Company in the late 1940s. Like most large aircraft of the era, it used radial engines, specifically the Pratt & Whitney R-4360 Wasp Major. The XF-12 was referred to as "flying on all fours" meaning: four engines, 400 mph (640 km/h) cruise, 4,000 mi (6,400 km) range, at 40,000 ft (12,000 m). The aircraft was designed to maximize aerodynamic efficiency. Although innovative, the jet engine and the end of World War 2 made it obsolete, and it did not enter production. A proposed airliner variant, the RC-2, was deemed uneconomical and cancelled before being built.

The Republic Aviation Corporation was an American aircraft manufacturer based in Farmingdale, New York, on Long Island. Originally known as the Seversky Aircraft Company, the company was responsible for the design and production of many important military aircraft, including its most famous products: World War II's P-47 Thunderbolt fighter, the F-84 Thunderjet and F-105 Thunderchief jet fighters, as well as the A-10 Thunderbolt II close-support aircraft.