An improvement of the B1 design, these boats were slightly heavier and of similar overall performance. Two further units of this type were canceled.
None of these six submarines survived the war. I-40 was sunk off Makin by destroyer USS Radford on 25 November 1943. I-41 sank after encountering destroyer escort USS Lawrence C. Taylor and aircraft from escort carrier USS Anzio off Samar on 18 November 1944. I-42 was torpedoed off the Palau Islands by submarine USS Tunny on 23 March 1944, while I-43 met the same fate at the hands of submarine USS Aspro 280 miles east of Guam on 15 February 1944. I-44 was sunk by aircraft from escort carrier USS Tulagi on 29 April 1945. Destroyer escort USS Whitehurst sank I-45 east of Surigao Strait on 29 October 1944.
| Units | 6 (none survived) |
|---|---|
| Ships | I-40, I-41, I-42, I-43, I-44, I-45 |
| Year(s) Completed | 1943 - 1944 |
| Displacement | 2,624 tons / 3,700 tons |
| Dimensions | 356.5 ft x 30.5 ft x 17 ft | Machinery | 2 diesels: 11,000 hp
electric motors: 2,000 hp |
| Speed | 23.5 knots / 8 knots |
| Range | 14,000 nm @ 16 knots |
| Armament | 6x533mm TT fwd + 1x14cm/50 cal. (17 Torpedoes) + one seaplane. |
| Max. Depth | 100 m (330 feet) |
| Crew | 114 officers and men |
