Additional 737-300s were obtained from Morris Air when its operations were absorbed by Southwest in 1994. Most of the 737-300 fleet was later retrofitted with slimmer-profile seats, increasing passenger capacity to 143. The new aircraft had a longer range and seated more passengers than the 737-200, 137 versus 122, and was substantially quieter inside, particularly behind the wing. Southwest was the first airline to operate the Boeing 737-300, which Boeing designed to meet the needs of the airline, as the 737-200 proved to have inadequate range and load-carrying capacity to fly new Southwest routes between Texas and the western United States. In September 1985, all five aircraft were returned, and Southwest never again operated the type. Around this time, Southwest also leased a fifth 727-200 from an unidentified lessor. In September 1983, Southwest leased two other 727-200 jets from People Express, adding two similar aircraft from the same airline in February 1984. This aircraft was returned in January 1980.

The first was leased from Braniff International Airways in August 1978 with Braniff providing training and maintenance support. Southwest operated six Boeing 727-200 aircraft between 19. Southwest retired its last active 737-200 from revenue service on Januhowever, one 737-200 was retained at Love Field until 2009 as a ground crew training aid.

The aircraft had flown for Southwest from March 15, 1984, to January 23, 2004. In early 2004, Southwest restored and donated the nose section of a retired 737-200, aircraft registration number N102SW, to the Frontiers of Flight Museum at Dallas Love Field. These were also the airline's first 737-200 Advanced series aircraft, with aerodynamic enhancements and greater range than the original 737-200. After obtaining six aircraft in this manner, Southwest made its first all-new 737 order from Boeing for four aircraft in June 1976. These aircraft were not originally ordered by Southwest, but rather were delivery slots taken over from Air California, Aloha Airlines and Pacific Southwest Airlines, including a lone 737-200 Combi which was later traded with a VASP 737-200. Southwest Airlines began revenue flights on Jusing three Boeing 737-200 aircraft, and operated the type exclusively during the airline's early history. These codes do not apply to recent aircraft types, including the 737 MAX 8 and 737 MAX 7. The Boeing customer code for Southwest Airlines is H4 for the Classic and Next Gen 737s. Southwest is the world's largest operator of the Boeing 737, and was the launch customer of the 737-300, 737-500, and 737-700. Since its inception, Southwest Airlines has almost exclusively operated Boeing 737 aircraft (except for a brief period when it operated a handful of leased Boeing 727-200 aircraft). Two Boeing 737-700s of Southwest Airlines at San Francisco International Airport

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