On March 3, 2012, United and Continental merged their passenger service systems, frequent-flier programs, and websites, which virtually eliminated the Continental brand with the exception of its logo. The merged airline began operating under a single air operator's certificate from the Federal Aviation Administration on November 30, 2011. The carriers planned to begin merging their operations in 2011. In late 2006, Continental Airlines and United began merger discussions, which concluded successfully in 2010. Finally, in early 2006, it emerged from court protection and resumed normal operations. It remained under court protection for more than three years. 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. The aircraft used, a McDonnell Douglas DC-10, therefore received the callsign " Executive One". In 1973, United Airlines became the first civil airline to carry an active President of the United States, when then-president Richard Nixon was on board of a regularly scheduled flight from Washington D.C to Los Angeles.
On March 28, 1931, UATC formed United Air Lines, Inc., as a holding company for its airline subsidiaries. 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. In 1927, William Boeing founded Boeing Air Transport to operate air mail routes under contract with the United States Post Office Department. VAL flew the first privately contracted air mail flight in the U.S. Continental Airlines is the successor to Speed Lines, which Varney had founded by 1932 and whose name changed to Varney Speed Lines in 1934. United traces its roots to Varney Air Lines (VAL), which Walter Varney founded in 1926 in Boise, Idaho. The 'U' Tulip, designed by Saul Bass, was the airline's icon from 1973 to 2010
Main article: History of United Airlines United Airlines' 1997–2010 logo. After its merger with Continental in 2010, United consistently ranks as one of the world's largest airlines it is currently first by the number of destinations served and third in terms of revenue and fleet size. In 1997, United became one of the five founding airlines of Star Alliance, of which it remains a member today. United was formed by the amalgamation of several airlines in the late 1920s, the oldest of these being Varney Air Lines, created in 1926 by Walter Varney who later co-founded the predecessor to Continental Airlines. Regional service is operated by independent carriers under the brand name United Express. United operates an extensive domestic and international route network across the United States and all six inhabited continents primarily out of its eight hubs, with Chicago–O'Hare having the largest number of daily flights and Denver carrying the most passengers in 2023. is a major American airline headquartered at the Willis Tower in Chicago, Illinois.