(Imperial War Museum photo, scanned from Polmar and Carpenter's "Submarines of the Imperial Japanese Navy")

Externally similar to the 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 four 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.