The twin-engine Boeing 737 was developed as a direct competitor of the McDonnell Douglas DC-9 but did not fly until about 2 years after the latter's introduction. The 737 has been produced in two versions, the 737-100 and the 737-200. Except for 30 units, all the aircraft produced have been the 737-200 version, which is a stretched, higher capacity, and heavier aircraft than the 737-100. The total number of orders for the 737 was 978 by mid-1982, and the type is currently being manufactured at the rate of 8 per month.
The two engines are mounted under the wings in a manner similar to that of the 707. The proximity of the engine nacelles to the under surface of the wing highlights the problem, previously mentioned, incurred by the underwing engine location as the size of the aircraft is reduced. The desire to avoid a high-mounted horizontal tail, and its possible stability problems, apparently was largely responsible for the choice of this engine location instead of the aft-fuselage-mounted arrangement. The horizontal tail is located on the fuselage below the root of the vertical tail. The 737 uses basically the same Pratt & Whitney engines as those employed on the Boeing 727 and the McDonnell Douglas DC-9.