The Lockheed L-193 Constellation II was a jet airliner design concept, designed between 1949 and 1953 with a swept wing and engines mounted at the tail. An airliner and tanker version were developed. The latter, in an aerial refueling competition initiated by the United States Air Force (USAF), won and was preferred over the Boeing KC-135 Stratotanker. Since the competing Boeing aircraft was ready to fly first, examples were ordered as an interim measure. They performed well enough that the L-193 was never ordered as a tanker, and airliner plans were dropped soon after.