Type A2 (Ko-Gata 2)
Ko-Gata A2 type. Built under War program of 1941. Development of Ko-Gata A1 type, but with decreased engine power and lower speed: at the expense of that endurance has been significantly increased.
I-12 was lost to unknown causes in January 1945, possibly an operational loss.
Type B3 (Otsu-Gata 3)
B1 and B2 types, these three boats had greatly reduced engine power, with enhanced range. The hangar opened forward to access the catapult mounted near the bow. Four additional units were canceled in 1943.
In 1945, I-56 and I-58 were modified to each carry six Kaiten, giving up their 14cm guns in the process. They also received snorkels at that time. Of these boats, I-58 achieved the most notable success by sinking unescorted heavy cruiser USS Indianapolis with conventional torpedoes on 30 July 1945, with heavy loss of life. I-58 was also the only member of this class to survive the war. I-54 was lost to destroyer escort USS Richard M. Rowell east of Surigao Strait. I-56 was lost to five destroyers and aircraft from escort carrier USS Bataan east of Okinawa on 18 April 1945. An improved variant, called the B4 type, was planned but never started. These would have been larger by about 200 tons, had a surface speed of 22.5 knots, and carried eight torpedo tubes forward (with 23 torpedoes). This class was planned to number 8 boats, but they were all canceled in 1943.
C2 type (Hei-Gata 2)
Hei-Gata C2 type. Under the 1941 War program building of 6 submarines under slightly advanced Hei-Gata C1 project was provided, the basic difference consisted in some smaller engine power and absence of midget submarine. Only three submarines were completed, orders on remaining I49-51 were cancelled in 1943. 4 more submarines it was supposed to build under the 1942 program (Nos 710-713), but works have not been begun. I-47 and I-48 were later modified to carry Kaiten. I-46 was sunk by destroyers USS Helm and USS Gridley off Leyte on 28 October 1944. Three American destroyer escorts put an end to I-48 near Yap on 23 January 1945. I-47 survived the war, and was scuttled off Goto Island in 1946.