Further products of the pre-war Japanese 'shadow' program, these two ships were designed ostensibly as 27,500grt passenger liners for the North American route, but about a year prior to launch they were reportedly taken over by the Navy for completion as carriers. Doubts have been expressed as to whether there was any intention to complete them as merchant ships, but the names Izumo Maru (for Hiyo) and Kashiwara Maru (Junyo) had apparently been assigned and the mercantile origins of the vessels are clearly evident: a clean hull, a good sheer forward, armor protection limited to l in over the machinery spaces, as a sort of afterthought, and subdivision within the hull which clearly would have been unacceptable in a purpose designed warship. A further problem was top weight. The relatively light scantlings were not entirely suited to a two-hangar arrangement, but a single hangar would have accommodated a quite inadequate air group. A compromise was achieved by severely restricting hangar height, to only 10ft 6in clear in the case of the lower level, which was barely sufficient for the A6M, but virtually impossible for any of the current range of strike aircraft, even were propellers to be swung.

There were two lifts, each about 45ft x 45ft, and the original assigned air group was made up of eighteen D3A, eighteen B5N and twelve A6M, plus two, five and three respectively as a reserve, although, in the event, rather fewer of the latter were embarked. Like Zuikaku, Junyo received concrete protection around her aviation fuel tanks before the end of her career, by which time she was fitted with Type 13 (on a tripod mast abaft the island) and Type 21 (atop the island and on the portside flight-deck edge, aft) radar antennae. By 1944, Junyo had a mix of D3A and D4Y dive bombers.

Junyo took part in the Aleutians attacks in June 1942, and prior to the Battle of Santa Cruz, during which she was engaged in strikes against the carrier Enterprise and the battleship South Dakota and cruiser San Juan, she assisted Hiyo in the despatch of the destroyer Meredith. She operated off Guadalcanal during the 'Tokyo Express' evacuations, January through February 1943. Junyo was seriously damaged during the Philippine Sea action and suffered further damage in December 1944 when she was hit by torpedoes from the submarines Redfish and Sea Devil off Nagasaki; she was never repaired and was laid up for the duration of the war, to be scrapped in 1947.