United Airlines is a major international airline headquartered in Chicago. It was founded in 1926 and has grown rapidly in the decades since. In 2010, United merged with the now defunct Continental Airlines to become the third largest airline in the world in terms of both fleet size and the number of routes it operates. The airline has eight major hubs across the US, with its busiest at Chicago-O’Hare Airport, operating flights to more than 200 domestic destinations and over 100 international airports spread across five continents. It is also a founding member of the world’s largest airline alliance, the Star Alliance, making it a great option to build air miles for frequent international fliers.