1944: I-36
and I-37
were converted to Kaiten carriers. I-36
could carry 6 Kaiten and I-37
could carry four.

The B1s
(Otsu-gata) were based on the K6As. However, their conning
towers were streamlined to improve their underwater performance. They
were designed for long-range scouting
and were equipped with an E14Y1
"Glen" seaplane. The latter
is credited with the only conventional air attack carried out against the U.S.
mainland during the Second World War, when a "Glen" from I-25 dropped incendiary bombs on
the forests of Oregon in an attempt
to start a forest fire.

The B1s were part of a balanced fleet of submarines to be completed by October 1941. The A1s were intended to direct scouting operations by B1 and C1
class submarines, the former specialized to locate targets and the
latter to carry out attacks. However, the only time the boats operated
in a manner that much resembled this prewar plan was in the attack on Pearl Harbor. Six B1s were initially ordered, and they were completed on schedule. Later, an additional 14 B1s were completed, suggesting the Japanese were pleased with the design. No other class of I-boat was built in such numbers.

The B1s
achieved some important successes against the Americans, including damage to Saratoga
by I-26 on 31 August 1942
that took her out of the Guadalcanal
campaign, and the sinking of Wasp by I-19 just two weeks later. The
same torpedo salvo that sank Wasp
also damaged North Carolina and sank O'Brien,
making it arguably the most devastating single torpedo salvo in
history.
I-26 later was responsible for
the sinking of Juneau after the cruiser
had been badly damaged in the Naval Battle of Guadalcanal, 12-13
November 1942.

Because of their excellent range, these boats were
sometimes used to exchange diplomats
and supplies between Japan and
the other Axis. For
example, I-30 left Kure in
May 1942 for Europe, arriving at Lorient on 6 August 1942, and returned
on 22 August 1922 but was sunk by a Britishmine off Singapore and was lost along with
the Würzburg radar she was carrying.

Towards the end of the war, two of the boats began
modifications to allow them to launch balloon bombs, but the work was not completed
before the surrender.