Boeing 707 ushering in the Jet Age. It established Boeing as one of the largest manufacturers of passenger aircraft, and led to the later series of airliners with "7x7" designations. The later 720, 727, 737, and 757 share elements of the 707's fuselage design. The 707 was developed from the Boeing 367-80, a prototype jet first flown in 1954. A larger fuselage cross-section and other modifications resulted in the initial-production 707-120, powered by Pratt & Whitney JT3C turbojet engines, which first flew on December 20, 1957. Pan American World Airways began regular 707 service on October 26, 1958. Later derivatives included the
Boeing 720 Although only 154 were built, the Boeing 720 and 720B were profitable due to the low research and development costs, being slightly modified versions of the 707-120. They were later replaced by the Boeing 727. The 720 is the only Boeing jet airliner not to follow the company's "7x7" naming formula (excluding former McDonnell Douglas airliners, such as the MD-80). Boeing announced its plans to develop a new version of the 707 in July 1957. It was developed from the 707-120 to provide for short- to medium-range flights from shorter runways. The model was originally designated 707-020 before being changed