The Boeing 737 is an American short- to medium-range twinjet narrow-body 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 ten passenger models with capacities from 85 to 215 passengers. The 737 is Boeing's only narrow-body airliner in production, with the 737 Next Generation (-700, -800, and -900ER) variants currently being built. Production has also begun on the re-engined and redesigned 737 MAX, which is set to enter service in 2017. Originally envisioned in 1964, the initial 737-100 made its first flight in April 1967 and entered airline service in February 1968 at Lufthansa. 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.