The Japanese modified existing weapons to use at Pearl Harbor
In 1940 the British learned of the need to modify their aerial torpedoes for use at Taranto. The shallowness of the harbor meant they had to limit the depth to which the weapon dove upon entry into the water. Yet they could not limit it too much; to do so meant the torpedo could broach, possibly breaking up as it bounced on the surface. The Japanese studied the British solution and found, though it had worked, it was unnecessarily complex. Throughout the summer of 1941, Japanese torpedo bombers practiced attacks at Kagoshima Bay, where launched torpedoes were recovered in nets to help determine the depths to which they plunged. A long-standing myth developed from these experiments; the Japanese solved the problem through the adoption of wooden fins affixed to the rear of the torpedo. There were wooden fins, though they were there for aerial stability, not depth control.
The wooden tailfins so often cited as the key to the success of the Japanese attack were in use, in various modifications, by the British, Italian, and US Navies, and had nothing to do with resolving the issue of depth control. That was achieved through the use of active roll-control mechanisms, gyroscopically controlled, installed in the torpedoes during modifications in 1941. The wooden tailfins have long attracted interest due to the simplicity of their design, but it was the torpedo engineers who solved the problem in 1941. Before the Pearl Harbor attack, each torpedo bomber crew had the opportunity to launch a simulated strike using the modified weapons. Though several missed (as they did later in combat) enough successfully struck their target to deem the weapon a success, and its use was authorized in the attack.