 In August 1941, Japanese aviators were practicing dropping torpedoes in the shallow waters of Kagoshima Bay, testing improvements in the Type 91 torpedo and developing tactics for the attack of ships in harbor.They discovered that the Nakajima B5N torpedo bomber could fly , somewhat faster than expected, without the torpedoes striking the bottom of the bay down.On December 7, 1941, the leading wave—40 B5Ns—used the tactic to score more than 15 hits during the attack on Pearl Harbor.