The long range and stealth of a submarine in combination with the speed and precision of an aircraft.

This would have seemed like a winning combination to any admiral worth his salt in the early 20th century, and sure enough, naval minds began experimenting with combining these two fairly modern tactical elements as early as World War I. The earliest German and British prototypes simply saw a floatplane resting on the deck of a surfaced submarine, which would then dive in order to release the aircraft for takeoff.

French Cruiser Submarine Surcouf

In the late 1920s, during the process of developing another type of hybrid ship (the cruiser submarine), floatplanes were added to the mix so they could serve as spotters for the submarine’s artillery. The difference in comparison to the most primitive concepts was that these new, specialized ships had a small hangar where a partially-assembled plane could be stored, allowing for submersion.

This new design caught on quick with the Japanese admiralty, and local shipbuilders soon began churning out large new combat submarines equipped with hangars for observation planes. Japan was the biggest believer in carrier subs, developing several types of varying sizes that were used to good effect during the Pacific War. So large was their investment in this type of ship that, in the end, the most widely used type of submarine by Japan during World War II was in fact designed to carry a plane. While the fairly standard observation and scouting role of their floatplanes was maintained, the Imperial Japanese Navy took a special interest in the possibility of using attack planes such as bombers or fighters in their place, effectively turning their submarine fleet into limited strike carriers.

Type B Submarine

The most numerous class of IJN submarine—Type B—was also the most successful in terms of Allied ship tonnage sunk. Also called the I-15 class, twenty of these subs were constructed between 1940 and 1943, being numbered between I-15 and I-39. This class of submarine was relatively fast while surfaced, travelling at speeds of around 23 knots, but when submerged, this number would reduce to just 8 knots. The Type B was also large enough to warrant the title of “cruiser submarine.” At 109 m, it was over 1.5 times the length of the Type VII U-boat, or roughly the length of twelve London double-decker buses placed end-to-end.

Type B Submarine I-26

These were fitted with a single 140 mm deck gun on the aft deck, two 25 mm Type 96 AA mounts, and six tubes for launching its complement of seventeen 533 mm Type 95 fast torpedoes. The aircraft they carried was the two-seater Yokosuka E14Y scout plane, kept in a compact forward hangar. This hangar was repurposed for several of these submarines towards the end of World War II to either accommodate an extra 140 mm gun, or to hold Kaiten manned torpedoes.

The original design had a small hangar forward of the conning tower that held a single floatplane. Naturally, due to lack of space, the aircraft would have to be pulled onto the deck to put its wings and floatation pontoons in place. Once complete, the plane would be hooked into a catapult mechanism which ran the length of the forward deck, and launched for a quick takeoff. After completing its mission, the plane would touch down in the water near the submarine and be hoisted back aboard and disassembled for storage.

Yokosuka E14Y “Glen” Floatplane

Despite its impressive numbers and capabilities (the Type B were even adapted to ferry supplies to island garrisons), the ship was far from perfect. When I-30 travelled to German-occupied France in 1942 as part of a so-called Yanagi exchange mission, experts from the Kriegsmarine’s U-boat fleet observed that besides being slow to dive and having an unimpressive operational depth of 100 m (as opposed to the Type VII U-boat’s 200+ m), she was noisy and vulnerable to both sonar and radar.

The Yanagi Missions

When Japan joined World War II, communication with her German allies became difficult. In order to exchange technology—such as the Japanese Long Lance torpedoes or German radar equipment—several secretive missions were carried out by submarines to make the 24,000 km voyage from Japan to France through Royal Navy-controlled waters. Out of a total of five voyages started, three successfully reached France, and only one returned to Japan. In turn, several Kriegsmarine U-boats attempted the opposite journey, with only one successfully reaching Japan.

One notable Type B submarine is I-25, which was responsible for two of the few direct (but ultimately unsuccessful) attacks on continental United States, when in 1942 she both shelled a naval fortification in Oregon and dropped an aerial bomb on a forest in the same state, in an attempt to start a wildfire.

Another Type B that deserves recognition is I-19, which was responsible for one of the most successful torpedo salvoes in history. On September 15, 1942, near the contested island of Guadalcanal, this submarine intercepted a convoy transporting US Marines. The submarine set its sights on an irresistible target—fleet carrier USS Wasp—and launched a full salvo of six torpedoes. Three of them slammed into the target’s starboard side in quick succession, sparking fires that quickly spread to the fuel tanks and magazines.

Two more torpedoes from the salvo slipped past the carrier, with one of them striking the bow of Sims-class destroyer USS O’Brien, and the other hitting battleship North Carolina. O’Brien succumbed to her wounds and sank days later while en route to getting repaired, and North Carolina was put out of action for months. Despite extensive efforts to control the fire aboard USS Wasp, the order had to be given to abandon ship. She sank 6 hours after the strike.

USS Wasp burns after being hit with three torpedoes.

I-400-class

The Japanese Navy weren’t content with having one airplane aboard their submarines. After the attack on Pearl Harbor, the primary objective of the Japanese Navy was to eliminate the U.S. Pacific fleet as quickly as possible and to delay American reinforcements at all costs, before the immense American industrial capacity could quickly overwhelm the Pacific Ocean with new warships. It was believed that this could be achieved by carrying out surgically precise strikes on key assets on the U.S. mainland. In so doing, the American population would surely be thrown into a panic, and countless U.S. military resources would have to be diverted to shoring up defenses along the Pacific coast.

The I-400 class submarines were the model chosen to carry out these attacks.

These submarines were massive for the standards of the time, being the largest submarines in service in World War II, but they were basically an oversized version (122 m in length) of the Type B class. As with that class, a single 140 mm gun was mounted on the aft deck, but the number of torpedo launchers was increased from six to eight. The number of anti-aircraft mounts was also increased, with four separate mounts. The hangar-catapult configuration worked similar to that of the Type B, but could now hold up to three two-seater Aichi M6A bombers. These modern aircraft were able to carry torpedoes or bombs, but the only machine gun was a rear-mounted flexible gun that was fired from the observer’s position.

The I-400s were expected to cross the Pacific Ocean undetected to launch a total of three bombers each. Several operations of this type were planned in 1945 when two of these submarines (I-400 and I-401) had been completed and commissioned. One such plan was to carry out an air raid on the locks of the Panama Canal—the main communication artery between the U.S. Atlantic and Pacific fleets—with ten submarine-borne aircraft. However, unfortunately for the Japanese Navy, these submarines were too little; too late, and the war came to an end before any of these strategies could really make a difference.

After the Japanese surrender, all three remaining I-400 class submarines came under the control of the United States Navy. Impressed by the size and concept behind them, they were closely studied by experts. Sadly, they were all scuttled shortly afterwards, out of fear of the Soviet Union getting hold of the technology.