The 17-shi was conceived as a one-mission attack bomber,14 no return, no deposit. Floats would be dispensed with (except for practice flights), not because a suicide mission was planned (those wouldn’t become standard for another three years) but to maximize the plane’s range and speed.‡ 15 After completing the mission, the aircraft’s crew would rendezvous with the I-400, ditch the plane, and swim to the sub to be picked up. Presumably nobody thought a second act could top bombing New York and Washington.
The design called for two flight crew seated tandem style: a pilot in front, who acted as bombardier, and an observer in back, who acted as radioman, navigator, and tail-gunner, and made the calculations necessary to set torpedo depth. Since the aircraft not only had to get to its destination but had to defend itself along the way, the initial spec called for two machine guns: a fixed, front-facing 7.7mm gun and a rear-facing 13mm Type 2 gun.16 The rear-facing machine gun was belt fed and handled 300 rounds.17 Flexibly mounted, it folded down when not in operation and locked against the plane’s starboard fuselage. When the gun was required, the observer swiveled his bucket seat 180 degrees to face backward and slid open the cockpit’s overhead canopy, automatically raising the gun into firing position—a pretty neat trick.
It soon became apparent that the 17-shi would have trouble outrunning an enemy fighter. The plane’s defensive armament wasn’t satisfactory either. In the event of an attack, it would require luck or damn fine piloting skills to survive. But nobody was complaining. Early versions of the Zero fighter had neither armor plating nor self-sealing fuel tanks in the event of a puncture—both standard features on Allied aircraft. Since Japanese pilots were famous for their courage, neither speed nor lack of armor seemed a problem.
Like many planes of its day, the special attack bomber had an aluminum alloy airframe, with wood limited to the instrument panel and wingtips.18 The control surfaces, namely the tail’s vertical and horizontal stabilizers, were fabric covered, yet the overall plane was solidly constructed.19 As one pilot who later flew the aircraft said, “It was the most cutting edge plane of its time,”20 and indeed, the special attack plane was destined to become not only Japan’s most advanced and complex aircraft of the war21 but its highest-performance floatplane in terms of payload and speed.
To keep things simple, Aichi looked at converting one of its own aircraft to do the job. The plane that made the most sense was the D4Y1 Suisei, a carrier-based dive-bomber (Allied code name: “Judy”). Aichi had recently begun mass production of the Suisei, and the company hoped it could be adapted for use aboard the I-400 with few modifications; Aichi probably realized that the best way to save money was to share the major assemblies of two aircraft.22 Unfortunately, the Suisei proved less adaptable as a sub-based plane. It may have been considered the IJN’s best single-engine bomber of the Pacific war,23 but it couldn’t be made to fit in the I-400’s hangar.§
Research, design, and testing of the I-400’s aircraft would consume Aichi for the rest of 1942 and most of 1943,24 and like everything else about the project, the schedule was ambitious. The I-400 subs were designed to carry two attack planes each. Given 18 subs, this meant constructing a minimum of 36 planes.25 But 36 planes were not enough, because trainers and replacement aircraft were also needed. The goal, then, was to construct a total of 78 planes, 44 in 1944 and another 34 in 1945.26 The size of the order confirms just how important the IJN viewed the Sen-toku squadron.
But the first step was to develop a design that addressed the complex balance among size, weight, range, speed, and payload yet still allowed the plane to be folded into a small enough package to fit inside a sub hangar. That the hangar was only 11 feet 6 inches in diameter27 made the job more challenging, since many propeller blades were longer than that. It wasn’t going to be easy.
AS MIGHT BE expected, the history of plane-carrying subs is an unusual one. At least six countries experimented with sub-plane combinations with varying degrees of success, but only Japan was able to carry the idea to fruition. Perhaps Japan’s success was due to understanding the benefits of a sub-plane combo better than other nations, or perhaps she had the greatest need. Whatever the reason, Japan was to spend nearly 20 years perfecting the combination, when other countries had long since given up.
Both the British and Germans tested a floatplane-submarine combination during World War I. The Germans did it first in January 1915, when a floatplane pilot teamed up with a U-boat captain he’d met socially to conduct aerial reconnaissance of an English town.28 The German High Command was not impressed, and no additional flights were made.
The HMS E-22 was another early example. A British E-class submarine commissioned in 1915, she carried two Sopwith Schneider seaplanes on deck to intercept and destroy German Zeppelins.29 The Admiralty had originally considered the idea of plane-carrying subs inhumane, but Zeppelin raids on London soon changed that notion. The first trial of the E-22 was conducted in the North Sea on April 24, 1916. Submerging the boat in a calm sea, the Sopwith Schneiders floated free and took off for England, proving the concept worked. The trial was not repeated because two days later the E-22 was torpedoed by a German U-boat. There were no survivors.
During the early days of sub-plane experiments, subs and floatplanes were used with a minimum of modification. The aircraft was carried on the sub’s deck, but there was no means of housing it and no way of retrieving the plane once she departed.
The United States began experimenting with its own version of a plane-carrying sub in 1923. The S-1 had a small on-deck storage container that housed a Martin MS-1.30 Unfortunately, before the plane could be launched, 16 man-hours were required to put it together.31 This made the Martin MS-1 about as popular as an unassembled toy on Christmas Eve. Once the aircraft was ready, the sub submerged enough to allow the plane to slip off its stern and take off in the water. The eventual launch must have been anticlimactic given the prep time.
The goal of the S-1 was to use its plane to scout ahead of the fleet and report back on the enemy’s position.32 But as submarine expert Norman Polmar explains, “The U.S. had enough dirigibles and carrier planes that they didn’t need to experiment with plane-based submarines.”33 It was no surprise then when the United States abandoned the program in 1927.
Britain picked up the gauntlet that same year with the M-2, also known as a “mutton boat” because its 12-inch gun and turret resembled a leg of lamb. The M-1’s first captain, Max Horton, was an early proponent of sub-borne airplanes. As a result, the M-2 had its gun removed and was refitted with a floatplane hangar, a compressed air catapult, and a crane for loading and unloading aircraft.34
The M-2 carried a Parnall Peto, which was a floatplane, but landing and recovering the aircraft in anything more than a force-two wind was nearly impossible.35 Though it may sound impressive, a force-two wind is a light breeze accompanied by small waves—in other words, a nice day at the beach.
Launching a Peto from the M-2 was an improvement over the time-consuming Martin MS-1, but not by much. Ten men had to crowd inside the tiny hangar to get the plane ready. They also had to wear waders and sea boots, because the hangar door was so close to the water, they often got soaked. The good news was that the Peto could be launched in under 12 minutes, a big improvement over the American S-1’s launch time. A Peto could also land, be recovered, disassembled, and stored with the M-2 returning to periscope depth in less than 11 minutes. Given this capability, it’s fair to say the M-2 was the first submarine fully capable of carrying, launching, and retrieving an airplane.36
The M-2 possessed an unfortunate design flaw, however. After the sub dove off England one January morning in 1932, the Royal Navy had to go looking for her. She was eventually found three miles off the coast in 100 feet of water. Her hangar door was open, as was the hatch leading from the hangar into the sub. Her entire crew of 60 had perished,37 as did Britain’s desire for further experimentation with plane-carrying subs. In the future, no submarine would have its hangar door placed so nea
r the waterline.
Though Germany, Britain, the United States, and Italy “passed” on the plane-carrying-sub concept, France was next, and her commitment was big. Named the Surcouf, after Robert Surcouf, who made his career as a “licensed” buccaneer attacking British ships,38 she was the world’s largest, most powerful submarine when commissioned in 1934. At 361 feet in length, she was one of the longest subs ever built until the I-400 class came along.
Surcouf’s size was dictated by the need for a stable weapons platform that could both launch a plane and have range enough to reach the enemy. (This was also an issue that the I-400s faced.) The French wanted a commerce raider to do to Germany what the U-boats had done to France during World War I. The French weren’t ones to stint on weapons systems either. As Richard Compton-Hall notes in Submarine Warfare: Monsters & Midgets: “The Surcouf had just about everything that could be crammed into a sub of the period,”39 including a watertight turret, out of which extended twin eight-inch guns. Below the waterline, the sub boasted four bow-mounted torpedo tubes supplemented with 22 torpedoes.‖ And of course she carried a floatplane. In other words, this baguette really packed a punch.
Surcouf even had restraints to imprison 40 POWs from the ships she sunk,40 a nice touch given Robert Surcouf’s second career transporting slaves. But the French were more interested in commerce raiding than in floatplanes and soon dispensed with her aircraft. When the Surcouf disappeared without a trace in February 1942, French interest went with it.
Japan’s interest in sub-borne aircraft was undeterred despite all this failure. In 1923 she purchased a Caspar-Heinkel U-1 seaplane from Germany41 and began holding sea trials to learn the trade. Japan had good reasons for developing plane-carrying subs. First, subs played an important scouting role in the IJN’s decisive battle strategy.42 The enemy fleet had to be located and reported on while still distant from Japanese shores, and subs were ideal for this role. But subs don’t handle scouting very well. If it’s true that a submarine has a large scouting range, it’s also true that it rides so low in the water that it has a restricted field of vision. And though a land-based aircraft has a much higher field of view, its scouting radius in the 1920s was limited to an hour or two. You could dramatically improve both the scouting area and the field of view if you built a submarine that effectively carried, launched, and retrieved an aircraft while at sea. The Japanese intended doing just that.
By 1925 Japan had developed her own prototype seaplane based on the Heinkel. Called the Yokosho 1-Go, she conducted tests using the I-21 as her host sub.43 Trials progressed enough by the fall of 1928 that the IJN was satisfied that a plane-carrying sub could serve a useful purpose.
In July 1932 the I-5 became the first Japanese submarine to carry an aircraft in a watertight deck hangar.44 When fleet maneuvers demonstrated that a sub-based reconnaissance plane could successfully find and report on the enemy, enthusiasm for the program grew. Assembling and disassembling the aircraft took too long, so the IJN continued pushing improvements.45 The I-6 followed in 1935. Built as an aircraft-carrying submarine from its inception (rather than modified later), the I-6 was followed two years later by the I-7 and a year after that by the I-8.46 All of these boats had their floatplane hangar on their aft deck behind the sail, a design that changed over time to favor the forward deck for hangar and catapult placement. The I-7’s and I-8’s storage and catapult arrangements were an improvement because the subs were larger than their predecessors.47 When it came to launching planes from a sub, bigger really was better.
By December 1936, Japan had withdrawn from the Washington and London naval treaties and began building submarines just as fast as she was able. Under the Third Fleet Replenishment Law, three new types of subs were planned, two of which, the Type A1 and B, carried a floatplane.48 Twenty new Junsen Type B boats were laid down in January 1938,49 followed by three new Junsen Type A1 boats laid down in January 1939.50 They were among the best subs in the world at the time51 and the only ones that carried airplanes.
Japan expected results from her aircraft-bearing submarines;52 that’s why plane-carrying subs were integrated into every major theater in which Japan’s sub force operated, including the attack on Pearl Harbor. By the start of war there were 11 frontline I-boats with floatplanes,53 and more were in the pipeline. By war’s end, Japan had built 41 such subs.a 54 Because of this commitment, no other nation had so much experience building and operating plane-carrying subs.
But the Japanese were soon to encounter an important strategic issue related to sub-plane combat: Which weapon took precedence? A sub is intended to hunt its enemy, while a plane is intended to scout. What this meant in practice is that each one’s mission interfered with the other. For example, before launching her scout plane, a sub would forgo attacking ships. This deterred combat operations. When her scout was returning, a sub was sometimes forced to give away her position to help the plane locate her. This ran counter to the sub’s purpose as a stealth weapon.
Sub commanders weren’t always happy subordinating their combat mission to reconnaissance, and the I-400 class only exacerbated the situation. The I-400’s primary job was to deliver aircraft to a set location. Attacking ships was now a secondary consideration. On the one hand, it was the next logical step in the evolution of sub-borne airplanes; yet it turned every conventional notion about submarines on its head. Which is why it’s not surprising that the Naval General Staff took an intermediate step in July 1942 when it instigated a mission where the seaplane rather than its host sub would be the offensive weapon. The mission was the first of its kind. It would be launched before the I-400’s keel had even been laid, and it would proceed using an existing plane that required little modification.
But what was most surprising was that eight months into the war, the Naval General Staff wasn’t just going to stand convention on its head—it was going to launch an aerial attack against the United States to prove that the idea could work. For Japan wasn’t satisfied with just using submarines to shell the American coastline. She was going to use an airplane-carrying sub to bomb the American mainland for the first time.
* Now called Penang.
† Squadron Leader Atsushi Asamura later said that the Aichi M6A1 cost 50 times more than a Zero fighter to build. Though it’s almost impossible to substantiate this number, the Seiran was clearly an expensive airplane. See Atsushi Asamura, interview, Rekishi Gunzou, no. 85, October 10, 2007, pp. 154–59.
‡ There is some controversy around whether the plane’s floats were designed to be jettisoned in flight (to conserve fuel or to increase speed to avoid enemy attack). Sato suggests jettisoning was considered an option during the planning stages. However, if the idea was considered, it did not get past the early design stage. See Sato, Maboroshi no Sensui Kubo, p. 62.
§ According to Robert C. Mikesh in Aichi M6A1 Seiran, had the adaptation worked, the plane would have been named Keisei.
‖ “None of them reliable,” Compton-Hall dryly reports.
a Hashimoto says in his book Sunk! that there were 11 aircraft-carrying subs at the beginning of the war. Orita notes in I-Boat Captain that there were 11 plane-carrying boats at Pearl Harbor; see Orita and Harrington, I-Boat Captain, p. 40. So it is reasonable to assume that the Sixth Fleet committed all its plane-carrying subs to the attack. Interestingly, these subs accounted for more than a third of the sub force deployed around Oahu (including Nambu in the I-17), which shows just how important plane-carrying subs were to the Japanese. Twenty-four additional plane-carrying subs were built as the war progressed, making for a total of 35. Hashimoto lists the plane-carrying subs at the start of war as including: the I-7, I-8, I-10, I-15, I-17, I-19, I-21, I-23, and I-26. These were followed by 19 boats numbered I-27 through I-45 as well as the I-54, I-56, I-58, I-11, and I-12. Of course, the I-400, I-401, I-13, and I-14 should also be added to this list, which increases the number to at least 39. See Hashimoto, Sunk!, p. 36, for details.
CHAPTER 6
PROOF OF CO
NCEPT
WHEN THE CAPTAIN OF THE I-25 HANDED WO NOBUO FUJITA A message summoning him to the Navy Ministry in Tokyo, Fujita was nervous. It was July 27, 1942,1 and the I-25 had recently returned from her third war patrol. Fujita had every intention of going on the sub’s next patrol. He respected the sub’s captain, Lt. Cdr. Meiji Tagami, and enjoyed flying her two-seater floatplane. He was in the middle of a much-deserved rest when he received the message, which was all the more unwelcome for failing to explain why he was being summoned.
It had been five months since Nambu shelled the oil facility near Santa Barbara and two months since the I-400 subs had been approved for production. Unfortunately, the IJN had suffered a huge loss at the Battle of Midway in June. Yamamoto remained set on taking the war to the American mainland though. Shelling the west coast had been a start, and the I-400s might well prove the finish. In the meantime, the IJN was searching for a second act.
Certainly, Fujita was more than just a proficient seaplane pilot. During the I-25’s second war patrol, he had completed six successful reconnaissance missions over enemy ports in Australia and New Zealand.2 But what could the Naval General Staff possibly want with him? He was just a pilot. They, on the other hand, were more akin to gods.
Operation Storm: Japan's Top Secret Submarines and Its Plan to Change the Course of World War II Page 6