There is a difference between motor torpedo boats (MTB) and torpedo boats. The first refers to small, fast boats, with an internal combustion engine, and armed with torpedoes. The latter, meanwhile, refers to warships that were, fundamentally, small destroyers. They were intended for coastal defence, anti-submarine work, and other tasks that required a significant warship but that a larger destroyer could not be spared for.

The Japanese built both torpedo boats and motor torpedo boats, though they generally preferred the former. The IJN may have believed that the MTB could play little part in its plans for a decisive victory in the open ocean. Equally, they had problems procuring the high-power engines that were needed to make an MTB truly effective. The various classes of MTB that the IJN did produce, such as the Type T-14, were slow compared to their foreign counterparts. Even so, they would build ~250 torpedo boats, especially towards the end of the war. I should also mention the explosive motorboats of the 'Shinyo' (IJN) and 'Maru-ni' (Imperial Japanese Army) types. These were intended to fulfil the same role as the motor-torpedo boat in terms of inshore coastal attack. However, rather than dropping or launching a projectile, they were, themselves, a projectile. Some 9000 of these craft were produced. They saw some action in the Philippines and around Okinawa, but to little effect.

Unlike the MTB, the torpedo boat could operate on the open ocean, and used steam engines that Japanese industry were experienced with producing. The torpedo boat also helped to fill in a perceived gap with the number of destroyers possessed by the IJN. The London Naval Treaty of 1930 limited Japan to 105,500 tons of destroyers, less than it felt it needed. However, the limits did not apply to ships displacing less than 600 tons. A ship that could carry out some of a destroyer's roles on a displacement of 600 tons would allow the IJN to make up the shortfall. The Chidori and Otori classes were built exactly to fit into this role. They could carry out coastal patrols and anti-submarine work, freeing up true destroyers for work with the fleet. However, they carried too heavy an armament for their size. After the Tomozuru (of the Chidori class) capsized in a gale in 1934, the topweight of the classes had to be heavily reduced to increase their stability. This involved reductions in armament and armour, the lowering of the bridge structure and the addition of ballast low in the ship.

It should be noted that torpedo boats were fairly common during WWII, especially in European navies. The French were probably the most enthusiastic adopters of the type, building four classes (45 ships) of them. The Italians followed suit, with the 32-ship Spica class of torpedo boats being constructed in 1934-37. These took advantage of the same 600-ton loophole that the Japanese did. The Germans also operated several classes, such as the Möwe and Type 24 classes. The British also built ships that exploited the loophole, though these were very different from the torpedo boats seen elsewhere. The Kingfisher-class sloops were intended to escort coastal convoys. They had a light gun armament, no torpedoes, and a heavy depth-charge armament, but didn't have the range to cross the Atlantic.