High Quality Content by WIKIPEDIA articles! 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—in this case, the Pratt Whitney R-4360 "Wasp Major." The aircraft was designed with maximum aerodynamic efficiency in mind. The XF-12 was referred to as an aircraft that was "flying on all fours" meaning: four engines, 400 mph cruise, 4,000 mile range, at 40,000 feet.Although highly innovative, the postwar XF-12 Rainbow was fated to compete against more modern jet engine technology and was not to enter production.