During World War II, and after it, Boeing was known for its military planes. Boeing had made very important bombers, like the propeller B-17 Flying Fortress and B-29 Superfortress and the jet B-47 Stratojet and B-52 Stratofortress. However, Douglas and other companies were far ahead of Boeing in the airliner market. The only successful airliners Boeing made before the 707 were the Boeing 314 Clipper and Boeing 307 Stratoliner. During 1949–1950, Boeing started looking at jets. Back then, aerial refueling (when a plane can get more fuel while flying) was being used a lot. The United States Air Force had ordered 800 KC-97 Stratotankers. Since planes were beginning to use jet engines, the USAF needed a jet tanker.