The Japanese developed new bombs for use against American ships
American battleships were sufficiently armored to resist the average aerial bombs which Japanese carrier-based aircraft were capable of carrying in 1941. For example, USS Arizona’s deck armor, upon which an aerial bomb was most likely to strike, was 5 inches thick. USS Oklahoma had 4.5-inch deck armor, USS California’s was 3.5 inches thick. The bombs in Japan’s inventory at the time were incapable of penetrating the armor, which protected the ships’ vital spaces. Though they could detonate on and within the superstructure, inflicting both damage and casualties, the likelihood of the ships being rendered inoperable from aerial bombs alone was small. Japan’s planners looked to modify an existing weapon which could penetrate the armor on American battleships, inflicting fatal wounds to the massive vessels. They found such a weapon in their existing stores of 16″ armored piercing artillery shells, carried on some of their newer battleships.
The Japanese created what they designated as the Type 99 Number 80 Mark 5 bomb, a modified 1,760-pound sixteen-inch armored piercing shell. Dropped from an altitude of 10,000 feet or higher, the bomb gained enough speed during its descent to pierce almost 6 inches of armor plate. It was one such shell, dropped from a level bomber, which struck Arizona, penetrated the armor protecting its forward magazines, and caused the explosion which destroyed the ship on the morning of December 7, 1941. Another such bomb struck USS West Virginia and failed to explode, one of a number of misfires which occurred with the weapon. The catastrophic explosion of Arizona remains one of the most famous, and infamous, images of the attack on Pearl Harbor. Despite that success for the Japanese, the modified shell bomb seldom saw combat use after December 7, having proven unreliable in combat.