Undated: This picture released by the US Navy shows a Japanese mock-up used to plan the attack on Pearl Harbor. Adm. Isoruku Yamamoto, Japanese naval attache in Washington, conceived the plan for the attack on Pearl Harbor in January 1941. The Japanese War College worked out the attack from this model, and in September 1941, Japanese carriers and their planes practiced bombing on an obscure island of Japan. Yamamoto had special fins placed on torpedos for the shallow waters of Pearl Harbor. December 7 was picked for the attack because Yamamoto knew naval commander Adm. Husband E. Kimmel kept his ships in Pearl Harbor on weekends.