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, cruise, range, at . 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. Development The original proposal from the United States Army Air Corps Air Technical Service Command in late 1943 was for a reconnaissance aircraft with a range of and a ceiling of . Its primary objective was high-speed overflights of the Japanese homeland and key enemy installations. During World War II, due to the range requirements of operating in the Pacific, existing fighters and bombers were being used but were poorly suited. The need existed for dedicated photo-reconnaissance aircraft with speed, range, and altitude capabilities beyond what was available. In August 1943, U.S. President Franklin D. Roosevelt's son, Colonel Elliot Roosevelt, commander of a Lockheed F-5 (a modified P-38 Lightning) "recon" unit, recommended the acquisition of a dedicated high-performance photo reconnaissance aircraft to provide pre- and post-strike target analysis intelligence as well as photo interpretation to better allow commanders to make decisions for bombing raids. Republic Aviation submitted the XF-12 and it was competing against the Hughes XF-11. Both were powered by the new P&W R-4360. The XF-12's first flight was made on 4 February 1946 and during flight testing, it reached an altitude of at , and demonstrated a range of , exceeding design criteria. The XF-12 could photograph in both daylight and night and under conditions of reduced visibility at high altitudes over long ranges and with great speed. This "flying photo lab" was capable of mapping broad stretches of the globe. Only two prototypes were built each of the XF-11 and the XF-12 were ordered into service by the U.S. Army Air Forces, as the requirement evaporated after World War II ended, while the cheaper off-the-shelf Boeing B-29 Superfortress and Boeing B-50 Superfortress could temporarily fill the role until the jet-powered Boeing RB-47 Stratojet entered service. The XF-12 was the fastest aircraft of its day to use