Airport is the second busiest airport in both the United States and the world. In 2008 the airport had 881,566 aircraft operations, an average of 2,409 per day. It is the largest hub of United Airlines (whose headquarters are in downtown Chicago) and the second-largest hub of American Airlines. It is operated by the City of Chicago Department of Aviation, associated with an umbrella regional authority.
The airport was constructed between 1942 and 1943 as a manufacturing plant for Douglas C-54s during World War II. The two million square-foot factory needed easy access to the workforce of the nation's then-second-largest city, as well as its extensive railroad infrastructure. Orchard Place was a small pre-existing community in the area and the airport was known during the war as Orchard Place Airport (hence the location identifier ORD). Douglas Aircraft Company's contract ended in 1945 and the facility was chosen by the City of Chicago to meet future aviation demands. Though its familiar three-letter code ORD still reflects the early identity of the airport, it was renamed in 1949, after Lieutenant Commander Edward "Butch" O'Hare, a World War II flying ace. The first commercial passenger flights were started there in 1955 and an international terminal was built in 1958, but the majority of domestic traffic did not move from Midway until completion of a 1962 expansion at O'Hare. The arrival of Midway's former traffic instantly made O'Hare the new World's Busiest Airport, serving 10 million passengers annually. In 1997, annual passenger volume reached 70 million; it is now approaching 80 million.