The Mackerel-class submarines were a pair of experimental prototype submarines built just prior to World War II and launched in 1940 and 1941. The two submarines were similar in size and capability to the S-class submarines built at the end of World War I, and had been ordered to test the feasibility of using mass production techniques to build small submarines. Until at least 1940 it was thought that mass production of fleet submarines would be impractical, and in any case small submarines could provide area defense for submarine bases. Once it became apparent that there would be sufficient production of the more capable Gato-class submarines, interest in the design waned and no additional small submarines were ordered. Submarine production standardized during the war on the Gato class an