The Boeing 737 is a narrow-body aircraft produced by Boeing at its Renton Factory in Washington. Developed to supplement the Boeing 727 on short and thin routes, the twinjet retains the 707 fuselage width and six abreast seating but with two underwing turbofans instead of four. Envisioned in 1964, the initial 737-100 made its first flight in April 1967 and entered service in February 1968 with Lufthansa. The lengthened 737-200 entered service in April 1968, and evolved through four generations, offering several variants for 85 to 215 passengers.

The 737-100/200 original variants were powered by Pratt & Whitney JT8D low-bypass engines and offered seating for 85 to 130 passengers. Launched in 1980 and introduced in 1984, the 737 Classic -300/400/500 variants were upgraded with CFM56-3 turbofans and offered 110 to 168 seats. Introduced in 1997, the 737 Next Generation (NG) -600/700/800/900 variants have updated CFM56-7 turbofans, a larger wing and an upgraded glass cockpit, and seat 108 to 215 passengers. The latest generation, the 737 MAX, 737-7/8/9/10 MAX, powered by improved CFM LEAP-1B high bypass turbofans and accommodating 138 to 204 people, entered service in 2017. Boeing Business Jet versions are produced since the 737NG, as well as military models.

As of April 2023, 15,591 Boeing 737s have been ordered and 11,395 delivered. Initially, its main competitor was the McDonnell Douglas DC-9, followed by its MD-80/MD-90 derivatives. In 2013, the global 737 fleet had completed more than 184 million flights over 264 million block hours since its entry into service. It was the highest-selling commercial aircraft until being surpassed by the competing Airbus A320 family in October 2019, but maintains the record in total deliveries.