The new generation of airliners that was developed in the USA in the 1930s included the Boeing 247, an all-metal 10-passenger low-winged monoplane, 60 of which were built for United Airlines.
Orders from other airlines were denied, resulting in the development of Douglas DC-2s and DC-3s and Lockheed Electras, which together monopolised US domestic air transport until 1940.
While some of these were used on international services by Pan American, the trans-ocean services used large flying-boats, culminating in the Boeing 314.
The larger landplanes first went into service in 1940, when Trans-World Airlines took delivery of five Boeing 307s used on transcontinental flights, and Pan American three for Miami-South American service. These were pressurized, but in 1941 that equipment was removed to improve range for use in support of the military effort.
Wartime service included North Atlantic ferry services, and routes via the South Atlantic as far as India, before the Boeing 307s were returned to their owners for commercial use for a brief period. Most were sold to French carriers, some later passed on to Indo-China as late as 1974. One is on exhibit at the Smithsonian in Washington.
Converted military unpressurised Douglas C-54s (and newly-built DC-4s) went into commercial service soon after the end of the war, followed soon after by pressurised Lockheed Constellations.
These were able to operate at higher altitudes and speeds, and reduced the need for refuelling stops on longer routes.
Douglas developed the DC-6, a larger, pressurised version of the DC-4, and nearly all the long-distance airlines used DC-6s or Constellations.
BOAC was the last major airline using flying-boats on Empire routes – the Boeing 314s used in wartime for the Atlantic crossings were replaced by Constellations in 1947, but Shorts flying-boats operated to Africa, the Orient and Australia into the 1950s.
For service on the North Atlantic BOAC was allowed to buy Boeing Stratocruisers, the double-deck passenger development of the Superfortress bomber, in order to compete with Pan American and American Overseas, which became available in 1948.