The Japanese eventually surrendered the I-401 and the other two remaining underwater aircraft carriers.

Lieutenant Commander Stephen L. Johnson had a problem on his hands; a very large problem. His Balao-class submarine, the Segundo, had just picked up a large radar contact on the surface about 100 miles off Honshu, one of Japan’s home islands, heading south toward Tokyo. World War II in the Pacific had just ended, and the ensuing cease fire was in its 14th day. The official peace documents would not be signed for several more days, on September 2, 1945, aboard the battleship USS Missouri in Tokyo Bay.

As Johnson closed on the other vessel, he realized it was a gigantic submarine, so large in fact that it first looked like a surface ship in the darkness. The Americans had nothing that size, so he realized that it had to be a Japanese submarine.

This was the first command for the lanky 29-year-old commander. He and his crew faced the largest and perhaps the most advanced submarine in the world. The Japanese I-401 was longer than a football field and had a surface displacement of 5,233 tons, more than three times the Segundo’s displacement. More troubling though was the sub’s bristling weaponry that included a 5.5-inch gun on her aft deck, three triple-barreled 25mm antiaircraft guns, a single 25mm gun mounted on the bridge, and eight large torpedo tubes in her bow.

The large sub displayed the mandatory black surrender flag, but when the Segundo edged forward, the Japanese vessel moved rapidly into the night. The movement and the continuing display of the Rising Sun flag caused concern. Johnson’s vessel pursued the craft that eventually slowed down as dawn approached. He brought his bow torpedo tubes to bear on the craft as the two vessels settled into a Mexican standoff.

Johnson and his crew had received permission by now to sink the reluctant Japanese vessel if necessary, but he realized he had a career-boosting and perhaps a technologically promising prize in his sights. Much depended on this untried American submarine captain and his wily opponent in the seas off Japan.

Little did Johnson know that the Japanese submarine was a part of the I-400 squadron, basically underwater aircraft carriers, and that the I-401 carried Commander Tatsunosuke Ariizumi, developer of the top-secret subs initially designed to strike the U.S. homeland in a series of surprise attacks. Ariizumi was considered the “father of the I-400 series” and a loyal follower of the emperor with years of experience in the Japanese Navy, so surrender was a disgrace he could not endure.

Johnson also had to contend with Lt. Cmdr. Nobukiyo Nambu, skipper of the I-401, who traced his combat experience back to Pearl Harbor. He now commanded the world’s largest submarine designed to carry three state-of-the-art attack planes in a specially built hanger located atop the vessel. These secret Aichi M6A1 planes were initially designed for “a second Pearl Harbor” or another surprise attack, possibly even against New York City or Washington, D.C. The I-400 series submarines were themselves full of technological surprises. They was capable of traveling around the world one and a half times without refueling, had a top surface speed of 19 knots (or nearly 22 miles per hour), and could remain on patrol for four months, twice as long as the Segundo.

Neither Nambu nor Commander Ariizumi readily accepted the emperor’s surrender statement when it was broadcast on August 15. The subsequent communiqués from Tokyo were exceptionally confusing, especially Order 114, which confirmed that peace had been declared but that all submarines were to “execute predetermined missions and attack the enemy if discovered.”

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The I-400: Weapon For a Second Pearl Harbor:

It was Admiral Isoroku Yamamoto, commander of Japan’s Combined Fleet and developer of the Pearl Harbor attack, who called for the construction of the I-400 series some three weeks after Pearl Harbor. The insightful Yamamoto, who was later killed when his aircraft was shot down by U.S. fighter planes, had toured the United States years before and had warned against a prolonged war with the highly industrialized United States.

However, once Japan was committed to war, he believed that submarine aircraft carriers dropping bombs “like rain” over major U.S. cities would surely cause the American people to “lose their will to fight.” A second surprise attack with even more to come would prove psychologically devastating to the Americans, Yamamoto believed, and perhaps would be the best way to get the Americans to sue for peace.

The Japanese had previous experience with plane-carrying submarines, but these were float planes used largely for reconnaissance. The float planes could be easily shot out of the sky by American fighters, and each submarine carried only one plane, hardly enough to prod the Americans to the negotiating table. Yamamoto always thought big, and he called for a submarine that could travel 40,000 nautical miles without refueling, or nearly four times the range of a Balao-class submarine like the Segundo.

In addition, the I-400 series submarines would carry 1,750 tons of fuel, food for four months at sea for its crew of 147 to 157 men, and two attack planes with a speed of 220 miles per hour and a range of some 600 miles. The hangar atop the sub would need to be at least 100 feet long to initially accommodate two aircraft and be strong enough to withstand the pressure of the deep and a possible attack from enemy planes when surfaced.

Yamamoto’s I-400-class submarines would displace some 6,560 tons submerged, about three times the displacement of the largest U.S. subs, and would be slightly more than 400 feet long, making them about the size of a small cruiser. Because one of the submarines was larger than a destroyer, it “wasn’t an exaggeration, then, to say that Yamamoto was asking for something akin to a small underwater aircraft carrier,” noted one observer.

Yamamoto called for the construction of 18 of the massive submarines carrying a total of 36 attack planes. The plan was rushed through the traditionally slow-moving Japanese naval bureaucracy by June 1942, with construction of the first five subs to begin in January 1943. The name of the special submarine class was abbreviated to Sen-toku.

The attack planes had to be designed from scratch. The need for speed, range and a decent sized bomb payload required tradeoffs. The wings had to be foldable to fit inside the tube, or hangar, atop the submarine. The design work, testing, and building of the plane was outsourced to the Aichi Aircraft Company.

To maximize range and speed, floats were removed from the planes. The crew would circle back after an attack, ditch the plane, and be picked up by the sub. Each plane had a pilot/bombardier in front and a radioman/navigator/tail gunner in the back. Initial plans called for a fixed, front-facing 7.7mm machine gun and a rear-facing 13mm Type 2 gun that was belt fed and handled 300 rounds.

The I-400 program did have its detractors in the heavily bureaucratic Imperial Japanese Navy. After the defeat at Midway in early June 1942, Japan became more focused on defending the homeland and far less on possible attacks on the U.S. mainland using the large submarines. The death of Yamamoto in mid-April 1943, just weeks after his 59th birthday, played further into the hands of conservative Japanese commanders. Cutbacks were ordered in the number of submarines to be built, although the I-400s’ striking ability was to be increased by adding a third attack bomber to the large vessels and adding a second plane to two smaller submarines, the I-13 and I-14.

Equally important, Japanese naval officials realized that with the loss of Guadalcanal, the nation’s defensive perimeter was at dire risk. New York and Washington were dropped as targets for the underwater aircraft carriers in favor of attacking the Panama Canal. A successful attack on the canal would choke the American war machine in the Pacific and buy time for the Japanese to regroup and strengthen the nation’s defensive perimeter.

“Storm From a Clear Sky”

The first test flight of the Aichi attack plane occurred on November 8, 1943. The plane, called Seiran or “storm from a clear sky,” reportedly handled fairly well as the world’s first sub-borne attack bomber. The Japanese began compiling limited available information on the heavily fortified Panama Canal. Their analysis showed that destroying the gate opening onto Gatun Lake would create a massive outpouring of water, destroying the other gates in its path while rushing toward the Caribbean Sea.

The United States had an estimated 40,000 troops defending the canal. The approaches were heavily mined, and there were major fortifications at Colon, Margarita Island, Toro Point, and Fort Sherman. The latter had 16-inch cannon with a range of some 25 miles. Antiaircraft batteries, radar stations, searchlights, nine aircraft bases, and 30 aircraft warning stations rounded out the canal’s defenses.