The ships were ordered as the fast luxury passenger liners "Izumo Maru" and "Kashiwara Maru" by "Nippon Yusen Kaisha" (Japan Mail Steamship Company-NYK) in late 1938. In exchange for a 60% subsidy of their building costs by the Navy Ministry, they were designed to be converted to aircraft carriers. To facilitate this process, they were fitted with a double hull, additional fuel oil capacity, provisions for the fitting of additional transverse and longitudinal bulkheads, installation of a longitudinal bulkhead to separate the turbine rooms, a strengthened main deck, more height between decks, rearrangement of the superstructure and passenger accommodations to facilitate the installation of aircraft elevators and hangars, more space for additional wiring, installation of a bulbous bow and the addition of aviation gasoline storage tanks fore and aft of the machinery spaces. NYK was only interested in a maximum speed of to save fuel, but the Navy wanted a maximum speed of no less than so they compromised by limiting the performance of the turbines to 80% of maximum power during peacetime.