United Airlines is one of the four big airlines of the United States, and like Delta Air Lines and American Airlines, it operates a diverse range of narrowbody and widebody aircraft. Among its inventory of around 1,000 mainline aircraft, it has one of the largest fleets of Boeing 767 aircraft. Of its 53 Boeing 767s, 16 are the Boeing 767-400ER variant, with the rest being Boeing 767-300ERs.