In 1931, the Japanese Navy developed the Type 91 torpedo, intended for a torpedo bomber to drop from a height of and a speed of . In 1936, the torpedo was given wooden attachments to the tail, nicknamed the "Kyoban", to increase its aerodynamic qualities—these attachments were shed upon hitting the water. By 1937, with the addition of a breakaway wooden damper at the nose, the torpedo could be dropped from and a speed of . Tactical doctrine determined in 1938 that the Type 91 aerial torpedo should be released at a distance of from the target. As well, the Japanese Navy developed night attack and massed day attack doctrine, and coordinated aerial torpedo attacks between land- and carrier-based torpedo bombers.