The Boeing C-137 Stratoliner is a retired VIP transport aircraft derived from the Boeing 707 jet airliner used by the United States Air Force. Other nations also bought both new and used 707s for military service, primarily as VIP or tanker transports. In addition, the 707 served as the basis for several specialized versions, such as the E-3 Sentry AWACS aircraft. The designation C-18 covers several later variants based on the 707-320B/C series. The C-137 should not be confused with the similar Boeing C-135 Stratolifter; although they share a common ancestor the two aircraft have different fuselages.