The Boeing 737 Classic is the second generation of Boeing's popular narrow-body airliners, developed from 1979 and comprising three main variants: the -300, -400, and -500. Its most notable innovation was the exclusive use of CFM56 high-bypass turbofan engines, which dramatically improved fuel economy and reduced noise, but required a distinctive non-circular air intake because the 737's low ground clearance forced the engine to be mounted ahead of the wing. The first variant, the -300, first flew in February 1984 and entered service with Southwest Airlines that December; it was followed by the stretched -400 (1988, with Piedmont) and the shorter -500 (1989, with Southwest), which served as a direct replacement for the earlier 737-200. The family offered a range of 2,060 to 2,375 nautical miles and seated between 110 and 168 passengers depending on the variant. Competing with the McDonnell Douglas MD-80 and later the Airbus A320, the Classic series was ultimately succeeded by the 737 Next Generation, with a total of 1,988 aircraft delivered between 1984 and 2000.