Before World War II, Mare Island had been in a continual state of build up. By 1941, the yard was expected to be able to repair and paint six to eight large naval vessels at a time. It employed 5593 workers at the beginning of 1939 which rapidly increased to 18,500 by May 1941, with a monthly payroll of $3,500,000(1941). Then came the attack on Pearl Harbor. During World War II, Mare Island specialized in building up the US Navy's submarine forces in the Pacific as well as building other Naval ships.