The XF-12 Rainbow was a four-engine, all-metal prototype monoplane designed by the Republic Aviation Company in the late 1940s. Like most large airplanes of the era, it used radial engines—in this case, the Pratt & Whitney R-4360 "Wasp Major"—to drive propellers.
The United States Army wanted a long-range photo-reconnaissance aircraft, and the Rainbow was Republic's attempt to provide it. With a cruise speed of over 450 statute miles per hour (725 kilometers per hour), it was certainly fast, but the Air Force (which had separated from the Army in 1947) decided that the Boeing RB-50 would fill the need instead. As a result, Republic cancelled its plans to build not only the XF-12 but the RC-2 civilian airliner version as well, leaving only the two prototypes. One crashed leaving Eglin Air Force Base, Florida, and the other ended up as a target on the Aberdeen Proving Ground, Maryland.