B1 Class, Japanese Submarines

Photograph of B1-class submarine

Wikipedia Commons



2198 tons standard displacement
2584 tons surfaced
3654 tons submerged


350'8" by 30'6" by 16'10"
106.88m by 9.30m by 4.90m

Maximum speed      

23.6 knots (surfaced)
8 knots (submerged)
Dive to 330 feet (100m)
Aircraft 1 seaplane


1 5.5"/50 gun
2 25mm/60 machine gun
6 21" torpedo tubes (17 torpedoes)
2-shaft diesel (12,400 shp) or electric (2000 shp)
800 tons diesel fuel
Range 14,000 nautical miles (23,000 km) at 16 knots surfaced
96 nautical miles (154 km) at 3 knots submerged
Modifications From 1941, some units replaced the seaplane facilities with a second 5.5" gun.

1944-7: Surviving units began receiving Type 13 air search radar and Type 22 surface search radar.

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 British mine 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.

Units in the Pacific:


North of Oahu      

Sunk 1942-11-2 off San Cristobal by McCalla
I-17 North of Oahu       Sunk 1943-8-19 off Noumea by Tui and aircraft
I-19 Northwest of Oahu       Sunk 1943-11-25 off Makin by Radford
I-21 Northwest of Oahu       Sunk 1943-11-27 by aircraft from Chenango off Tarawa
I-23 Northwest of Oahu       Missing from 1942-2-15 off Oahu
I-25 North of Oahu       Sunk 943-8-25 by Patterson in the New Hebrides.
I-26 Between Hawaii and San Francisco       Sunk 1944-10-28 off Surigao by Gridley and Helm
I-28 Completed 1942-2-6 (Mitsubishi-Kobe)       Torpedoed 1942-5-17 off Truk by Tautog
I-27 Completed 1942-2-24 (Sasebo) Sunk 1944-2-12 by Paladin and Petard
I-29 Completed 1942-2-27 (Yokosuka) Torpedoed 1944-7-26 north of Luzon by Sawfish
I-30 Completed 1942-2-28 (Kure) Mined 1942-1-13 off Singapore
I-32 Completed 1942-4-26 (Sasebo)       Sunk off Wotje by Manlove and aircraft
I-31 Completed 1942-5-30 (Yokosuka) Missing from 1943-5-13 off Attu
I-33 Completed 1942-6-10 (Mitsubishi-Kobe)       Sunk accidentally on 1944-6-13 off Matsuyama
I-34 Completed 1942-8-31 (Sasebo)       Torpedoed 1943-11-13 off Penang by Taurus
I-35 Completed 1942-8-31 (Mitsubishi-Kobe)       Sunk on 1943-11-22 off Tarawa by Frazier
I-36 Completed 1942-9-30 (Yokosuka)      
I-38 Completed 1943-1-31 (Sasebo)       Sunk 1944-11-12 off Yap by Nicholas
I-37 Completed 1943-3-10 (Kure)       Sunk 1944-11-19 off Kossol Roads by Conklin and McCoy Reynolds
I-39 Completed 1943-4-22 (Sasebo)       Sunk 1943-11-26 by Boyd

Photo Gallery


Wikimedia Commons

B1 class ONI 220 page


B1 class ONI 220 page



Carpenter and Polmar (1986) (accessed 2009-2-23)
Jentschura, Jung, and Mickel (1977)

Nakagawa (1997)

Prange (1981)

Ward (2001)
Worth (2001)

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