There were a lot of military assets involved in the Pearl Harbor attack, from the Mitsubishi A6M Zero fighters to the vessels lined up at Battleship Row. Working in Japan’s favor was not just the fact that nobody at the American naval base knew they were coming, but also some of the armament at its disposal. Though the ships at Pearl Harbor were moored in shallow waters and conventional torpedoes would have been useless, the forces of the Imperial Japanese Navy had planned for this and, months prior to December 7th, 1941, devised a plan to get around the natural depths of the harbor.
To get to the ships moored at Battleship Row, the Japanese would need an explosive that would level out before crashing into the bottom of the harbor, only about 40′ deep. Typical torpedoes would not level out in time to strike the battleships, so Japanese researchers went to the drawing board for a device that would mimic the motions of an acrobat high-diver in shallow water, something that would be able to stabilize quickly.
By adding a fin to torpedoes, the Japanese created the secret weapon to a successful attack. The Type 91 was equipped with a wooden fin attachment, a stabilizer, and an acceleration system that controlled the rolling motions, making it one of the most advanced torpedoes of its time. Initial production on the Type 91 began in 1931, but it wasn’t until 1941 that the second revision proved the ability to clear shallow water and successfully strike the ships along Battleship Row.
These anti-ship bombs were a device that no one stationed at Pearl Harbor would ever forget. Deployed by the D3A1 and B5N2, the Type 99 was responsible for the initial strikes on the USS Arizona. It was one of these 1,763-lb explosives that destroyed the battleship, sending her—along with 1,177 of her men—to an eternity on the bottom of the harbor floor.
Though the battleships were the Japanese main target, the B5N2 Kates and the D3A1 Vals were equipped with Type 98 land bombs to maximize damage to the harbor and the American forces. During the first wave of the attack, the 98 land bomb was dropped by the Vals to destroy American aircraft on the ground at Hickam, Ford Island, and Wheeler airfields. The aim was to keep the Americans grounded to prevent the possibility of a counterattack.
During the second wave, the Kates replaced the Vals and continued bombarding the air bases on the island.