On December 20, 1929, Pan Amaerican awarded a contract to build three aircraft as the S-40, with deliveries starting mid-1931. The S-40 featured four engines and a two-step all metal hull, divided into seven watertight compartments. Almost sixty feet in length, accomodation was provided for up to thiry-two passengers and the added luxury of a smoking room with three chairs. Sikorsky selected a monoplane wing with stabalising floats mounted on outrigger booms. When built the Sikorsky S-40 was the largest amphibian in the world.
Flight testing of the S-40 began in early 1931, and it was delivered to Pan American the following October, as soon as the ATC certificate was granted.
Named “American Clipper” and piloted by Charles A. Lindbergh, the first S-40 inaugurated the Miami-Canal Zone route on 19 November 1931. "American Clipper" pioneered Pan American World Airways mail and passenger routes around the Caribbean and to South America. It was joined by the second, ‘Caribbean Clipper’, later the same year, then by the the third, ‘Southern Clipper’, in early 1932.
The three S-40s established regular airline service between the US east coast and South American destinations such as Rio de Janerio and Buenos Aires, operated without their amphibious gear to improve range as needed. The S-40 carried 40 passengers over distances of 500 mi. at speeds up to 115 mph.
During 1935, after all three had their engines upgraded to super-charged Hornets, their designation was changed to S-40-A. By the end of 1939 the S-40As had been withdrawn from Pan American service. Caribbean Clipper later served with the Navy as a navigational trainer and is said to have amassed a total of 13,000 flying hours before being scrapped in 1944.