The Boeing 737 is a short- to medium-range twin-engine narrow-body jet airliner. Originally developed as a shorter, lower-cost twin-engine airliner derived from Boeing's 707 and 727, the 737 has developed into a family of nine passenger models with a capacity of 85 to 215 passengers. The 737 is Boeing's only narrow-body airliner in production, with the -600, -700, -800, and -900ER variants currently being built. A re-engined and redesigned version, the 737 MAX, is set to debut in 2017. Originally envisioned in 1964, the initial 737-100 flew in 1967 and entered airline service in February 1968. Next the lengthened 737-200 entered service in April 1968. In the 1980s Boeing launched the -300, -400, and -500 models, subsequently referred to as the Boeing 737 Classic series. The 737 Classics added capacity and incorporated CFM56 turbofan engines along with wing improvements. In the 1990s Boeing introduced the 737 Next Generation with multiple changes including a redesigned wing, upgraded cockpit, and new interior. The 737 Next Generation comprises the four -600, -700, -800, and -900ER models, ranging from 102 ft (31.09 m) to 138 ft (42.06 m) in length. Boeing Business Jet versions of the 737 Next Generation are also produced. The 737 series is the best-selling jet airliner in the history of aviation. The 737 has been continuously manufactured by Boeing since 1967 with 7,457 aircraft delivered and 3,044 orders yet to be fulfilled as of January 2013. 737 assembly is centered at the Boeing Renton Factory in Renton, Washington. Many 737s serve markets previously filled by 707, 727, 757, DC-9, and MD-80/MD-90 airliners, and the aircraft currently competes primarily with the Airbus A320 family. There are, on average, 1,250 Boeing 737s airborne at any given time, with two departing or landing somewhere every five seconds.