The 737-300 was launched in 1981 by both USAir and Southwest Airlines becoming the first model of the 737 Classic series. The aircraft has a typical capacity of 128 passengers in a two-class configuration (137 seats in a one-class coach seating configuration). The 300 series remained in production until 1999 when the 1,113th and last aircraft was delivered to Air New Zealand on December 17, 1999.
Various modifications have been made to aircraft previously in service. The 737-300 can be retrofitted with Aviation Partners Boeing winglets. The 737-300 retrofitted with winglets is designated the -300SP (special performance). Used passenger -300 aircraft have also been converted to freighter versions. The Lockheed Martin CATBird is a modified 737-300 with the nose of a Lockheed Martin F-35 Lightning II, a pair of canards, and (inside) an F-35 cockpit; to be used to flight test the F-35's avionics suite.
In December 2008, Southwest Airlines selected Boeing to retrofit its 737-300s with new avionics, in order to improve commonality with its 737-700s, as well as to support the Required Navigation Performance initiat.