when the u.s. joined world war ii in december 1941, it was thought to be a luxury to fly. the war meant that government and military officers needed to fly long distances. planes like pan am's 14 flying boats and twa's five boeing 307s were used for this. more fuel tanks were added to allow it to fly further. military boeing 307s were called c-75s. before world war ii stopped them being made, 10 307s had been made for airlines. twa flew between new york and los angeles for 18 months until the army bought their planes. twa changed their 307s to c-75s in january 1942. these were the only american planes which could cross the atlantic with cargo until the douglas c-54 skymaster was made in november 1942.