The Lockheed R6V Constitution[n 1] was a large, propeller-driven, double-decker transport aircraft developed in the 1940s by Lockheed as a long-range, high-capacity transport and airliner for the U.S. Navy and Pan American Airways. Only two of the aircraft were ever built, both prototypes. Although these two planes went into service with the Navy, the Constitution design ultimately proved underpowered and too large for practical airline use at the time.