The Boeing 737 is a short to medium range, single aisle, narrow body jet airliner. Originally developed as a shorter, lower cost twin engine airliner derived from Boeing's 707 and 727, the 737 has nine variants, from the early -100 to the most recent and largest, the -900. Series -600 through -900 are still being produced.

First envisioned in 1964, the 737 entered service in 1968. Forty years later it has become the most ordered and produced commercial passenger jet in the world. It is Boeing's last surviving narrow-body airliner currently in production, sometimes serving markets previously filled by 707, 727, 757, DC-9 and MD-80/90 airliners. The 737 has been continuously manufactured by Boeing since 1967 with over 8,000 ordered and 5,857 delivered as of 2008. There are over 1,250 737s airborne at any given time, with one departing or landing somewhere every five seconds on average.

The 737 models can be divided into three generations, including nine major variants. The "Original" models consist of the 737-100, 737-200/-200 Advanced. The "Classic" models consist of the 737-300, 737-400, and 737-500. The "Next Generation" variants consist of the 737-600, 737-700/-700ER, 737-800, and 737-900/-900ER. Of these nine variants, many feature additional versions such as the T-43 which is a modified Boeing 737-200 used by the United States Air Force (USAF)

In the late 1980s, Boeing marketed the Boeing 77-33 jet, a business jet version of the 737-300. The name was short-lived. After the introduction of the next generation series, Boeing introduced the Boeing Business Jet (BBJ) series. The BBJ1 was similar in dimensions to the 737-700 but had additional features, including stronger wings and landing gear from the 737-800, and has increased range (through the use of extra fuel tanks) over the other 737 models. The first BBJ rolled out on 11 August 1998 and flew for the first time on 4 September.

On 11 October 1999 Boeing launched the BBJ2. Based on the 737-800, it is 5.84 m (19 ft 2 in) longer than the BBJ, with 25% more cabin space and twice the baggage space, but has slightly reduced range. It is also fitted with auxiliary belly fuel tanks and winglets. The first BBJ2 was delivered on 28 February 2001.

Boeing's BBJ3 is based on the 737-900ER. The BBJ3 has 1,120 square feet (104 m2) of floor space, 35% more interior space and 89% more luggage space than the BBJ2. It has an auxiliary fuel system, giving it a range of up to 4,725 nautical miles (8,751 km).