[GeronL]

and carried a fourth disassembled

[colorado tanker]

Did some googling and discovered this was a pet project of Yamamoto, who thought a fleet of these things could bring Doolittle-like raids to major US cities. After he was killed the project was scaled back considerably.

High command planned an attack on the Panama Canal, the theory being that would choke US supplies and ship movements. After Okinawa fell and the Home Islands were the next target the Panama plan was abandoned and a new plan hatched for kamikaze attacks on US carriers readying attacks on the Home Islands in the Ulithi anchorage. That never happened, either. One of the boats was discovered running on the surface and was sunk. Very interesting boat that had no impact on the war.