The 400-, 500- and 600-series were already used by missiles and other products, so Boeing decided that the jets would bear 700-series numbers, and the first would be the 707.[4] Boeing had studied developments of its existing Model 367 (the KC-97 Stratofreighter) incorporating swept wings and podded engines; and chose to build the 367-80, which retained little of the KC-97 except the upper fuselag