Boeing had been studying short-haul jet aircraft designs and wanted to produce another aircraft to supplement the 727 on short and thin routes. Preliminary design work began on May 11, 1964, and Boeing's intense market research yielded plans for a 50- to 60-passenger airliner for routes 50 to 1,000 mi (80 to 1,609 km) long. Lufthansa became the launch customer on February 19, 1965, with an order for 21 aircraft, worth $67 million in 1965, after the airline received assurances from Boeing that the 737 project would not be canceled. Consultation with Lufthansa over the previous winter resulted in an increase in capacity to 100 seats.
On April 5, 1965, Boeing announced an order by United Airlines for 40 737s. United wanted a slightly larger airplane than the original 737. So Boeing stretched the fuselage 91 centimeters (36 in) ahead of, and 102 cm (40 in) behind the wing. The longer version was designated 737-200, with the original short-body aircraft becoming the 737-100.
Detailed design work continued on both variants at the same time. Boeing was far behind its competitors when the 737 was launched, as rival aircraft BAC-111, Douglas DC-9, and Fokker F28 were already into flight certification. To expedite development, Boeing used 60% of the structure and systems of the existing 727, the most notable being the fuselage cross-section. This fuselage permitted six-abreast seating compared to the rival BAC-111 and DC-9's five-abreast layout. Design engineers decided to mount the nacelles directly to the underside of the wings to reduce the landing gear length and kept the engines low to the ground for easy ramp inspection and servicing. Many thickness variations for the engine attachment strut were tested in the wind tunnel and the most desirable shape for high speed was found to be one which was relatively thick, filling the narrow channels formed between the wing and the top of the nacelle, particularly on the outboard side. Originally, the span arrangement of the airfoil sections of the 737 wing was planned to be very similar to that of the 707 and 727, although somewhat thicker. However, a substantial improvement in drag at high Mach numbers was achieved by altering these sections near the nacelle. The engine chosen was the Pratt & Whitney JT8D-1 low-bypass ratio turbofan engine, delivering 14,500 lbf (64 kN) thrust. With the wing-mounted engines, Boeing decided to mount the horizontal stabilizer on the fuselage rather than the T-tail style of the Boeing 727.
The 737's main landing gear under the wings at mid-cabin rotates into wells in the aircraft's belly. The legs are covered by partial doors, and "brush-like" seals aerodynamically smooth (or "fair") the wheels in the wells. The sides of the tires are exposed to the air in flight. "Hub caps" complete the aerodynamic profile of the wheels. It is forbidden to operate without the caps, because they are linked to the ground speed sensor that interfaces with the anti-skid brake system. The dark circles of the tires are clearly visible when a 737 takes off, or is at low altitude.
737s are not equipped with fuel dump systems. The original aircraft were too small to require them, and adding a fuel dump system to the later, larger variants would have incurred a large weight penalty. Boeing instead demonstrated an "equivalent level of safety". Depending upon the nature of the emergency, 737s either circle to burn off fuel or land overweight. If the latter is the case, the aircraft is inspected by maintenance personnel for damage and then returned to service if none is found.
Engines on the 737 Classic series (300, 400, 500) and Next-Generation series (600, 700, 800, 900) do not have circular inlets like most aircraft. The 737 Classic series featured CFM56 turbofan engines, which yielded significant gains in fuel economy and a reduction in noise over the JT8D engines used on the -100 and -200, but also posed an engineering challenge given the low ground clearance of the 737. Boeing and engine supplier CFMI solved the problem by placing the engine ahead of (rather than below) the wing, and by moving engine accessories to the sides (rather than the bottom) of the engine pod, giving the 737 a distinctive non-circular air intake.
The wing also incorporated a number of changes for improved aerodynamics. The engines' accessory gearbox was moved from the 6 o'clock position under the engine to the 4 o'clock position (from a front/forward looking aft perspective). This side-mounted gearbox gives the engine a somewhat triangular rounded shape. Because the engine is close to the ground, 737-300s and later models are more prone to engine foreign object damage (FOD). The improved CFM56-7 turbofan engine on the 737 Next Generation is 7% more fuel-efficient than the previous CFM56-3 in the 737 classics. The newest 737 variants, the 737 MAX family, are to feature CFM International LEAP-1B engines with a 1.73 m fan diameter. These engines are expected to be 10-12% more efficient than the CFM56-7B engines on the 737 Next Generation family.
The primary flight controls are intrinsically safe. In the event of total hydraulic system failure or double engine failure, they will automatically and seamlessly revert to control via servo tab. In this mode, the servo tabs aerodynamically control the elevators and ailerons; these servo tabs are in turn controlled by cables running to the control yoke. The pilot's muscle forces alone control the tabs. The 737 is the only currently produced passenger aircraft this size or larger that can operate completely without hydraulics. For the 737 Next Generation, a six-screen LCD glass cockpit with modern avionics was implemented while retaining crew commonality with previous generation 737.
Most 737 cockpits are equipped with "eyebrow windows" positioned above the main glareshield. Eyebrow windows were a feature of the original 707 and 727. They allowed for greater visibility in turns, and offered better sky views if navigating by stars. With modern avionics, they became redundant, and many pilots actually placed newspapers or other objects in them to block out sun glare. They were eliminated from the 737 cockpit design in 2004, although they are still installed in military variants, or civilian aircraft at customer request. These windows are sometimes removed and plugged, usually during maintenance overhauls, and can be distinguished by the metal plug which differs from the smooth metal in later aircraft that were not originally fitted with the windows.
Blended winglets are available as retrofits and in production on newer 737 aircraft. These winglets stand approximately 8 feet (2.4 m) tall and are installed at the wing tips. They help to reduce fuel burn (by reducing vortex drag), engine wear, and takeoff noise. Overall fuel efficiency improvement is up to five percent through the reduction of lift-induced drag.
As of July 2008 the 737 features carbon brakes manufactured by Messier-Bugatti. These new brakes, now certified by the Federal Aviation Administration, weigh 550–700 lb (250–320 kg) less than the steel brakes normally fitted to the Next-Gen 737s (weight savings depend on whether standard or high-capacity brakes are fitted). A weight reduction of 700 pounds on a Boeing 737-800 results in 0.5% reduction in fuel burn.
A short-field design package is available for the 737-600, -700, and -800, allowing operators to fly increased payload to and from airports with runways under 5,000 feet (1,500 m). The package consists of sealed leading edge slats (improved lift), a two-position tail skid (enabling greater protection against tail strikes that may be caused by the lower landing speeds), and increased flight spoiler deflection on the ground. These improvements are standard on the 737-900ER.
The 737 interior arrangement has changed in successive generations. The original 737 interior was restyled for the 737 Classic models using 757 designs, while 777 architecture was used for the debut of the Next Generation 737. Designed using Boeing's new cabin concepts, the latest Sky Interior features sculpted sidewalls and redesigned window housings, along with increased headroom and LED mood lighting. Larger pivot-bins similar to those on the 777 and 787 have more luggage space than prior designs. The Sky Interior is also designed to improve cabin noise levels by 2–4 dB. The first 737 equipped with the Boeing Sky Interior was delivered to Flydubai in the fourth quarter of 2010. Malaysia Airlines, TUIFly, United/Continental Airlines, Pegasus Airlines and Turkish Airlines have also taken delivery of Sky Interior-equipped 737s.
- 737 Original series
- 737 Classic series
- 737 Next Generation
- 737 MAX
- Boeing Business Jet