United Airlines, Inc. (commonly referred to as United) is a major American airline headquartered at the Willis Tower in Chicago, Illinois. United operates a large domestic and international route network spanning cities large and small across the United States and all six inhabited continents. Measured by fleet size and the number of routes, it is the third-largest airline in the world after its merger with Continental Airlines in 2010. United has eight hubs, with Chicago–O'Hare being its largest in terms of passengers carried and the number of departures. It is a founding member of the Star Alliance, the world's largest airline alliance with a total of 28 member airlines. Regional service is operated by independent carriers under the brand name United Express. The United brand name was established by the amalgamation of several airlines in the late 1920s, the oldest of these being Varney Air Lines, which was founded in 1926. History United traces its roots to Varney Air Lines (VAL), which Walter Varney founded in 1926 in Boise, Idaho. Continental Airlines is the successor to Speed Lines, which Varney had founded by 1932 and whose name changed to Varney Speed Lines in 1934. VAL flew the first privately contracted air mail flight in the U.S. on April 6, 1926. In 1927, William Boeing founded Boeing Air Transport to operate air mail routes under contract with the United States Post Office Department. In 1929, Boeing merged his company with Pratt & Whitney to form the United Aircraft and Transport Corporation (UATC) which then set about buying, in the space of just 28 months, Pacific Air Transport, Stout Air Services, VAL, and National Air Transport, as well as numerous equipment manufacturers at the same time. On March 28, 1931, UATC formed United Air Lines, Inc., as a holding company for its airline subsidiaries. In December 2002, due largely to the post-9/11 dropoff in air travel, as well as to poor relations between the corporation and one of its key labor unions, the International Association of Machinists, United Airlines filed for bankruptcy. It remained under court protection for more than three years. This enabled it to cut costs ruthlessly. Finally, in early 2006, it emerged from court protection and resumed normal operations. In late 2006, Continental Airlines and United had preliminary merger discussions. On April 16, 2010, those discussions resumed. The board of directors of Continental and UAL Corporation agreed on May