In the middle of the 1970s, the USAF required new tanker aircraft for the near future and thus, the ATCA (Advanced Tanker/Cargo Aircraft) program was started. The reasons for this program were that the large fleet of KC-135 Stratotanker (over 700 units), showed during the Vietnam War, not to be sufficiently capable to refuel all bombers and reconnaissance aircraft in South East Asia, but as well as to refuel long range bombers and transport aircraft over the United States. So the United States Air Force wanted to have a larger and more flexible aircraft for this role. These aircraft should be not too expensive and easy too maintain, to reach this goal, it was decided to modify already in service being wide-body aircraft, like the Boeing 747, the Lockheed L-1011 TriStar and the McDonnell Do