United Airlines, Inc., commonly referred to as United, is a major American airline headquartered in Chicago, Illinois. It is the world's largest airline when measured by number of destinations served, and the second largest when measured by scheduled passenger-kilometres flown. United operates a comprehensive domestic and international route network, and has significant presence in the Asia-Pacific region. In the late 1920s, just prior to the use of the United Airlines name, The Boeing Company, currently one of the world's largest aircraft manufacturers, operated a predecessor airline. United was previously known as United Air Lines (UAL).
United is a founding member of Star Alliance, the world's first and largest global airline alliance. Regional service is operated by independent carriers under the brand name United Express. Its main competitors are American Airlines, Delta Air Lines, and Southwest Airlines.
United operates out of nine airline hubs located in Houston, Chicago, Newark/New York, Denver, San Francisco, Washington, D.C., Los Angeles, Guam, and Tokyo. George Bush Intercontinental Airport in Houston is United's largest passenger carrying hub handling 16.6 million passengers annually with an average of 45,413 passengers daily, while Chicago-O'Hare is its largest hub in terms of daily departures. The company employs over 88,500 people while maintaining its headquarters in Chicago's Willis Tower (formerly known as Sears Tower). Through the airline's parent company, United Continental Holdings, it is publicly traded under NYSE: UAL with a market capitalization of over $18 billion as of September, 2014. United operates maintenance bases in Cleveland and Orlando in addition to the maintenance locations located at United's hubs.