United Airlines has emerged as a prominent operator of the Boeing 737 fleet, which includes both the 737NG (Next Generation) and 737 MAX series. Currently, United's fleet consists of nearly 210 737 MAX aircraft and approximately 330 737NGs of various models, placing it as the third-largest 737 operator globally, following Southwest Airlines and Ryanair.
The Boeing 737, a narrowbody aircraft, announced its first delivery on February 10, 1968, with Lufthansa as its launch customer. United's significant 737NG fleet originated mainly from its merger with Continental Airlines in 2010. Prior to the merger, Continental had placed its first 737NG order in 1996, eventually receiving over 200 airframes. During the two airlines' discussions in 2008, Continental was the fifth largest U.S. carrier, operating over 300 aircraft, and had firm decisions made for future additions of aircraft.
In the merger with United, Continental handed over 350 aircraft, including 240 Boeing 737s with a significant portion being 737NG models. Continental's fleet addition also comprised Boeing 767s and 777s. As part of the merger arrangement, United took control of Continental's upcoming Boeing 787 Dreamliner order, planned for service commencement in the subsequent year.