When did passenger airlines begin?

Before airplanes, DELAG's Zeppelin airships carried paying passengers in Germany from 1910. The first scheduled airplane service was organized by the St. Petersburg–Tampa Airboat Line, which connected those two Florida cities across the bay for about four months in 1914: a 23-minute flight, one passenger per trip and a $5 fare. The first customer, former mayor Abram Pheil, paid $400 at auction for the inaugural seat. The service closed when the tourist season ended.

World War I left thousands of aircraft and pilots available. On 25 August 1919 Aircraft Transport and Travel opened the first daily international route, London–Paris, with De Havilland biplanes derived from bombers. That year saw the founding of KLM, today the oldest airline still operating under its original name, and SCADTA in Colombia, the origin of Avianca. Junkers unveiled the F 13, the first all-metal aircraft designed for passengers, with an enclosed cabin for four people. Flying remained expensive and unreliable, and mail contracts financed many routes.

The 1933 Boeing 247 and, above all, the 1935 Douglas DC-3 set the formula: all-metal monoplane, retractable gear, two engines and around twenty seats. The DC-3 was the first aircraft on which an airline could turn a profit from passengers alone, without postal subsidy. The pressurized cabin arrived with the Boeing 307 in 1940, and the jet age began with the de Havilland Comet in 1952, followed by the Boeing 707 in 1958, which made the airplane the standard way to cross the Atlantic.