United Airlines has a huge Boeing 737 fleet of almost 540 aircraft consisting of both the 737NG (Next Generation) and 737 MAX series. With its 737 MAX fleet currently at around 210 airframes, the carrier has approximately 330 Boeing 737NGs, comprised of the -700, -800, -900 and -900ER models. In fact, as it stands, United is the third-largest operator of the Boeing 737 family behind Southwest Airlines and Ryanair, demonstrating the narrowbody’s importance to United’s fleet.