Type D submarine

The , also called or was a type of the 1st class submarine in the Imperial Japanese Navy serving during the Second World War. The type name, was shortened to .
The , also called was different from the "I-361" class, however since the "I-373" was a development form of the "I-361" class, this article describes both of them.
Construction.
After the Battle of Midway the IJN immediately planned a transport submarine. The type was based on the "U 155 Deutschland". Her duties were transportation of troops (110 men, 10 tons freight and two landing craft) in the areas where the enemy had air superiority. Later the demands for her were changed in sequence. The final demands were 65 tons in the hull and 25 tons on the upper deck (freight only). In the beginning the IJN did not intend to arm these boats with torpedoes. Later, after strong demands from the front commanders, it was decided to arm them with torpedoes for self-defense. The "I-372" class was designed as a tanker submarine based on the "I-361" class. They were not allowed to be loaded with torpedoes.
Service.
In 1944, the submarines were tasked with transport missions between from mainland Japan to remote islands. They had little success and suffered great losses. Of the 13 submarines, only four survived the war.
Kaiten missions.
In 1945, several submarines were converted to be "Kaiten" mother ships and assigned to the suicide attack operations for the . Their deck guns were removed and fitttings for five "Kaiten"s were installed on their decks.
Class variants.
The "Type-D" submarines were divided into four classes: 
"I-372" class.
Project number S51B. She was going to become a lead ship of the "Modified Type D" submarines ("I-373" class), at first. However, the IJN wanted a submarine as soon as possible. She was built as a tanker submarine according to revised "I-361" drawings.
"I-373" class.
Project number S51C. Improved model of the "I-361" class. Furthermore, the IJN was planned reinforced model of the "I-373" class, also. However all of them were cancelled.