thumb|FedEx Express McDonnell Douglas MD-11.FedEx Express is the world's largest airline in terms of number of aircraft and in terms of freight tons flown. thumb|Ryanair Boeing 737-800 shortly after take-off. Ryanair is the world's largest airline in terms of number of international passengers carried. An airline provides air transport services for passengers or freight, generally with a recognized operating certificate or license. Airlines lease or own their aircraft with which to supply these services and may form partnerships or alliances with other airlines for mutual benefit.

Airlines vary from those with a single airplane carrying mail or cargo, through full-service international airlines operating hundreds of airplanes. Airline services can be categorized as being intercontinental, intra continental, domestic, or international and may be operated as scheduled services or charters.

The first airlines

Early Development

thumb|TWA Douglas DC-3 in 1940. The DC-3, often regarded as one of the most influential aircraft in the history of commercial aviation, revolutionized the aviation industry. Tony Jannus conducted the United States' scheduled commercial airline flight on 1 January 1914 for the Saint Petersburg-routes, Braniff Airways, American Airlines, Delta Air Lines, United Airlines (originally a division of Boeing), Trans World Airlines, Northwest Airlines, and Eastern Air Lines, to name a few.

Passenger service during the early 1920s was sporadic: most airlines at the time were focused on carrying bags of mail. In 1925, however, the Ford Motor Company bought out the Stout Aircraft Company and began construction of the all-metal Ford Trimotor, which became the first successful American airliner. With a 12-passenger capacity, the Trimotor made passenger service potentially profitable. Air service was seen as a supplement to rail service in the American transportation network.

At the same time, Juan Trippe began a crusade to create an air network that would link America to the world, and he achieved this goal through his airline, Pan American World Airways, with a fleet of flying boats that linked Los Angeles to Shanghai and Boston to London. Pan Am and Northwest Airways (which began flights to Canada in the 1920s) were the only U.S. airlines to go international before the 1940s.

With the introduction of the Boeing 247 and Douglas DC-3 in the 1930s, the U.S. airline industry was generally profitable, even during the Great Depression. This trend continued until the beginning of World War II.

Development since 1945

thumb|In October 1945, the American Export Airlines became the first airline to offer regular commercial flights between North America and Europe. Shown here is Am Ex Boeing 377 Stratocruiser in 1949. As governments met to set the standards and scope for an emergent civil air industry toward the end of the war, it was no surprise that the U.S. took a position of maximum operating freedom. After all, U.S. airline companies were not as hard-hit as European and the few Asian ones had been. This preference for "open skies" operating regimes continues, within limitations, to this day.