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.

Minimizing drag was a primary consideration throughout the design of the XF-12. Many features came from Republic's experience with fighter aircraft. Unusually, no compromises to the aerodynamics were made in the shape of its fuselage. Aviation Week was quoted as saying "the sharp nose and cylindrical cigar shape of the XF-12 fulfills a designer's dream of a no compromise design with aerodynamic considerations." Air for engine intakes, oil coolers and intercoolers was drawn through the leading edge of each wing between the inboard and outboard engines. This reduced drag compared to using individual intakes for each component. In addition, because the air was taken from a high-pressure area at the front of the wing, this provided a ram air boost for increased power at high speeds, and more effective cooling of the oil and intercoolers. The intakes made up 25% of the total wingspan and were extensively wind tunnel tested. After being used, the air was ducted toward the rear of the nacelle, to provide thrust. The entire engine nacelle was nearly as long as a Republic P-47 Thunderbolt. Research showed that a force roughly equivalent to 250 hp (190 kW) was generated by each engine exhaust during high speed cruise while at altitude.[7] Each engine featured twin General Electric turbochargers at the rear of the nacelle and for brief bursts of additional power, water-methanol injection.

For its reconnaissance role, the XF-12 had three photographic compartments aft of the wing. One vertical, one split vertical, and one trimetrogon each using a 6 in (150 mm) Fairchild K-17 camera. For night reconnaissance, the XF-12 had a belly hold which accommodated 18 high-intensity photo-flash bombs to be ejected over the target. All bays were equipped with electrically operated, inward retracting doors designed for minimum drag and camera lenses were electrically heated to prevent frost build-up. The XF-12 also carried complete darkroom facilities to permit developing and printing the film while still airborne augmented by adjustable storage racks to handle any size of film container and additional photo equipment. This allowed immediate access to the intelligence after landing without the usual processing delay.

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.

​