The early exchanges of the Pacific War very quickly established the ascendancy of air over heavy gun power and one immediate result of this in Japan was the cessation of all work on capital ships. This decision directly affected the construction of the Yamato-class battleships, one of which had been completed by mid 1942 with a second almost ready for sea and a third whose hull was virtually complete on the slips. The last offered the prospect of an enormous platform for air operations and one, moreover, that would be prodigiously armored. It also suggested an internal volume that could accommodate vast quantities of fuel and ordnance, and it was this capacity which led to the idea that the converted vessel might be very usefully employed as a support ship for replenishing fleet combat carriers without their having to return home; replacement aircraft would also be embarked, but they would not form a combat air group for the carrier herself.

The battleship hull was essentially retained in toto, with its armour and underwater protection, although the belt was reduced from 15.7in to 8.1in except along the magazines where it kept its original thickness. The principal horizontal protection consisted of a 7.5in armor deck, but sources differ as to whether this was at main deck level, as in the original battleship design, or raised to hangar deck (ie former upper deck) level. The hangar itself was a single-storey unit some 550ft long, built up over the forecastle deck, open for much of its length (roller shutters for enclosure), served by two lifts, and surmounted by a 3.lin armored flight deck. The island, sponsoned out to starboard, was generally similar to though considerably larger than that evolved for the Hiyos, with a single large uptake angled over to starboard. Types 13 and 22 radars are believed to have been fitted. Shinano would have been equipped with an air group of 40-50 planes (the initial scheme was revised during conversion), stowed forward in the hangar, but the bulk of the capacity would be given over to replacement aircraft for fleet carriers and forward land bases. However, the ship never entered service: while moving to Kure for final fitting out she was struck by four torpedoes from the US submarine Archerftsh (29 November 1944). Damage control facilities in the form of pumps and watertight doors had not been completed, and flooding spread from the starboard engine room and empty petrol bunkers, sinking the vessel some seven hours after being hit.