Aboard the submarine, the aircraft was mounted at four attach points to a dolly that rolled on a rail to the inclined catapult, then served as the launching shuttle during the the catapult’s 69-foot power stroke until the aircraft separated from the dolly and flew away. The launching dolly was equipped with an articulated support structure so that the aircraft would be in a low-silhouette squat position while in the hangar tube and, when rolled out of the tube onto the catapult track, would be tilted nose up for higher angle of attack during launch. The aircraft was launched in the same manner even when the floats were attached to the aircraft. The floats straddled the catapult ramp without touching the sides of the ramp or the deck surface below.
As the aircraft design evolved early into adapting the detachable floats, a somewhat subtle but potentially significant consequence developed. Both the chief and assistant test pilots, Tadashi Funada and Yukitaka Murakami, were trained and experienced solely in floatplanes flying reconnaissance missions, not in bombers. So were the squadron commander, Lieutenant Atsushi Asamura, and the rest of the pilots assigned. The inexperience of the floatplane pilots in dive bombing and torpedo launching, along with the limited time available for developing the additional skills, reportedly led the unit to abandon the use of torpedos. Further, in training and in mission planning, only a shallow dive was employed in attacks to get the best accuracy attainable within the pilots’ limited expertise. This is ironic because the Seiran was one of the few aircraft in the world designed to be capable of both torpedo attack and dive bombing—besides being the first and perhaps the last sub-launched manned attack aircraft.
The Pacific war developed into the drawn-out match that Yamamoto had feared: It was a contest of national wills, armed might, and, more critically to Japan, industrial capacity. The main theater of war soon contracted to the southwestern rim of the Pacific and drew ever closer to Japan’s mainland. On a tour of Japanese naval bases undertaken in a desperate attempt to boost the morale of the troops holed up in the scattered last bastions of the crumbling Empire, Yamamoto’s airplane was shot down by an American P-38. With the death of the brilliant strategist, the Imperial Japanese Navy lost the force of his vision to “reach” the U.S. mainland. But the rapidly changing war situation may already have rendered the operation futile.
In 1944, war production in mainland Japan was in a shambles. The Japanese were fighting to defend what was left in Sokoku (ancestor’s land). Performance and expediency, certainly not engineering elegance, had become the operative words of the military and industry. Yet at the Aichi Aircraft Company, under the watchful eyes of the Kugisho, production of the M6A1 reached 28. Four I-400-class sen-toku were being built. The I-400 and I-401 entered service with their aircraft, and the I-402 was converted to a transport submarine. A fourth one was destroyed by a U.S. air raid on its shipyard.
On December 15, 1944, Number 631 Air Squadron was established, and on December 30, Number 1 Submarine Squadron. These units were intended to make up the special task force to carry out the Panama Canal attack. As U.S. forces drew closer to Japan, headquarters strategists began to abandon the Holy Grail of Panama, instead planning to send the attack group on a more essential mission: to strike at the U.S. naval task force readying for the inevitable assault on the Japanese mainland. Japan’s intelligence marked Ulithi Atoll at the western end of the Caroline Islands as the target. The idea was not received well by the Yamamoto traditionalists, particularly Captain Ryunosuke Ariizumi, who, as one of the staff analysts in the Naval Operations Command, had endorsed the original Yamamoto proposal and was now assigned as commanding officer of the Number 1 sub squadron as well as commander of the entire sen-toku attack force. Ariizumi earnestly argued against reducing Yamamoto’s strategic vision to a mere tactical stroke and pleaded to high command, but to no avail. The target of the sen-toku force mission was changed officially from the Panama Canal to Ulithi Atoll on June 25, 1945, when the order was passed to the units in training for the mission to return to home port.
The Ulithi operation consisted of two elements: Operation Hikari (Light) ordered I-13 and I-14 submarines to transport two Saiun (Nakajima C6N1, Allied code name Myrt) advanced reconnaissance aircraft aboard each ship to Truk, an island base still held by Japan approximately 900 miles east of Ulithi in the Carolines, to conduct reconnaissance flights to verify the presence of the U.S. fleet. The Saiuns’ high-speed, high-altitude, long-range performance were called for in this mission to ensure success of the valuable Seiran attack force—to be launched in unrecoverable float-less configuration. The I-13 hangar designed for two big Seirans was large enough for this big, three-seat reconnaissance airplane equipped with a 2,000-hp Homare engine and developed as a carrier-based aircraft. The two ships, having replaced the Seirans with Saiuns, left Maizuru on July 2. The I-13 never made it to Truk; it was reported missing and later confirmed sunk by a U.S. destroyer.
The plan for Operation Arashi (Storm) was, true to the Seiran name, to rain storms of attack on the U.S. fleet from the southern sector, where no Japanese force was known to exist. Six Seirans, three aboard Ariizumi’s I-401 flagship and three more on the I-400, thus constituted the final one-shot attack force.
There was a solemn send-off ceremony on July 19 in Maizuru—a ceremony that had become an increasingly frequent routine in the Japanese navy and army air forces in the last days of the war, as pilots of Tokko Tai (Special Attack Corps) readied themselves for their final sortie. The 12 crewmen of the six Seirans were each presented with a dagger in an unlacquered wooden scabbard—a samurai warrior’s personal weapon that has not drawn blood—as a symbol of prayer for success in their last and highest mission of honor. The daggers were sent by the sixth fleet commander in chief and personally handed out by Ariizumi. These gifts expressed an unspoken expectation that each samurai would not hesitate to make the final dive of his aircraft and dash himself and his comrade into the enemy target to ensure the success of the mission in defense of the “ancestor’s land.” That these missions were tokko (special attack), called “kamikaze” by Westerners, was solemnly understood by the flight crew, the commanders, and the nation. It may not have been written in the order of the battle, but it was implicit in the prevailing doctrine of battle, the sacred ritual, and the mounting urgency of the homeland defense. For the first time in the kenkon itteki strategy that had produced the one-mission aircraft designed for maximum effect, the mission became a truly one-way tokko attack. The world’s only sub-aircraft carrier attack force left for its final mission from Ominato, the hastily assigned home port at the northern tip of Honshu.
Published accounts, recently confirmed, of the preparation for the Ulithi attack alluded to the red hinomaru (the sun circle of the Japanese national flag) marking of the Seirans being painted over with the star markings of the U.S. forces. Another account, that the aircraft were repainted in the silver color predominant among U.S. aircraft, has not been confirmed in the recollections of those present. In a 1999 interview with National Air and Space Museum restoration specialist Robert McLean, Murakami, the Kugisho test pilot who had performed the final flight check of the Seirans at Maizuru base, remembered his astonishment and shame when he saw the U.S. star on the folded wing of a Seiran on the deck of an I-400 submarine. He could hardly believe that the Imperial Japanese Navy would resort to such deception. Asamura, the 631st squadron’s flight leader, told McLean that “the Ulithi attack was a ‘one in thousands’ chance.” If mistaking the Seirans for P-51s could cause the American defenders to hesitate for one second, he continued, at least one or two aircraft may be able to get through to hit an American warship. “Thus the intense desire to succeed in the slim-chance attack overrode the shame of the acknowledgedly ‘cowardly’ tactic,” Asamura said.
On August 15, 1945, even as the elements of the force headed toward their rendezvous point for attack, Emperor Hirohito announced Japan’s surrender to the Allied powers. The submarines were instructed to disarm and return to their home ports, cruising on the surface and flying a black triangular flag in accordance with the surrender agreement. The special force’s main weapon systems, the revered Seiran M6A1s, were scuttled—launched, with wings folded, from the I-400s’ catapults to sink into the depths of the Pacific.