Far-sighted Japanese naval experts predicted that in a war with America, Japan would need to keep supply lines to forward American bases always in doubt. Aircraft-equipped submarines (SSV) would be able to travel in loose packs and support each other in scouting and attacks on shipping. These huge subs (the world's largest) had tremendous range and proved to be quite upsetting to the Allies when they surfaced in unexpected places and backwater war zones.

I-400's Seiran aircraft attacked and sank the old French BB Ocean off the Andamans early in the war. In a surprise attack on Dutch Harbor, an American AA cruiser was sunk in port by I-409. A Dutch airfield was bombed and an Australian cruiser damaged by I-407. The heavy carrier Ranger was caught in a web of torpedoes from a "wolfpack" of all seven of this class operating together. Their most daring feat was the effective, but temporary disabling of the Panama Canal by 21 sub-launched bombers and the sinking of the Chilean CV Almirante Cochrane. By April of 1944, however, good ASW tactics by American aircraft had eliminated all of the I-400 series submarines .