The first passenger intercontinental flights were based on European countries with colonial empires:
KLM (Netherlands), SABENA (Belgium), and Air France all commenced services in the late 1920s from their home countries to outposts in Africa, and together with Imperial Airways of Britain soon started flying to the Middle East and Orient. The short ranges of available aircraft meant frequent stops, and by the time in the late 1930s passenger service was available to the Orient and Australia, the journey from Europe included multiple overnights at en-route points.
Pan American Airways from the late 1920s, through a combination of intense diplomatic and commercial pressure, built up a massive network throughout the Caribbean and South America
Oceanic crossings started over the North Pacific with several island stops between Honolulu and Hong Kong by Pan American, with flying boats operating once weekly. The limiting sector for available load was San Francisco-Honolulu, a flight of 2,398 miles. The South Pacific route started in 1938, also with several island stops between Honolulu and Auckland: this initially operated every two weeks.
In 1939, Pan American’s Boeing 314 flying boats operated once weekly from San Francisco to Hongkong and two weekly flights from Port Washington, New York, one to Southampton and one to Marseilles, while Imperial Airways flew from Southampton three times weekly to Kisumu, Kenya, two continuing to Durban, South Africa; five weekly to Calcutta, India, three continuing to Sydney, Australia, and did not publish a schedule for North Atlantic services.
KLM flew three times weekly from Amsterdam to Batavia, two continuing to Sydney as KNILM flights. SABENA operated once weekly from Brussels to the Congo. Air France flew once a week from Marseilles to Saigon and on to Hongkong, and a mail service on the South Atlantic. Lufthansa also operated a weekly mail service to South America
North Atlantic operations had started in 1939, when Pan American and Imperial flying boats commenced New York-Southampton service with intermediate stops, after preliminary mail services commenced. Pan Am also started service to France via Lisbon. The outbreak of war limited service to neutral Eire and Portugal almost immediately, while Pacific flights continued until the entry of Japan closed all civilian flights. Also ceased was the Qantas route to Singapore: later the Kangaroo Hop (Perth-Ceylon) was in operation, then the world’s longest overwater route.
During the war, Imperial Airways became BOAC and, to avoid Northern winters, operated some North Atlantic services via West Africa and South America, as did Pan American. Other US airlines with suitable equipment operated under contract to the military as well. Pan American’s South American contacts opened up the route from the USA via Brazil to West Africa and on to India.
One of the effects of wartime was the widespread construction of landplane airstrips capable of handling the longer-ranged DC-4s and Constellations that became available to airlines in time to recommence scheduled intercontinental traffic.
Flying boats continued on some routes until about 1949, but by that time there were multiple competing commercial landplane services on most routes.