Originally planned as the ocean liner Izumo Maru (出雲丸) in 1939, she was purchased by the Navy Ministry in 1941 for conversion to an aircraft carrier.

She was sunk by a gasoline-vapour explosion caused by an American torpedo hit during the Battle of the Philippine Sea on 20 June 1944 with the loss of 247 officers and ratings, about a fifth of her complement.

The ship was ordered as the fast luxury passenger liner Izumo Maru by Nippon Yusen Kaisha (Japan Mail Steamship Company) in late 1938.

In exchange for a Navy Ministry subsidy of 60% of her building costs, she was designed to be converted to an aircraft carrier in the event of war.

The ship carried enough fuel oil to give her a range of 11,700 nautical miles (21,700 km; 13,500 mi) at 18 knots (33 km/h; 21 mph).

[8] The ship's primary armament consisted of a dozen 12.7-centimetre (5 in) Type 89 dual-purpose guns in twin mounts on sponsons along the sides of the hull.

Hiyō was also initially equipped with eight triple mounts for 2.5 cm Type 96 light AA guns alongside the flight deck.

[9] This was the standard Japanese light AA gun during World War II, but it suffered from severe design shortcomings which rendered it a largely ineffective weapon.

[13] The ship was assigned to the Second Carrier Division of the 1st Air Fleet after commissioning and became flagship of Rear Admiral Kakuji Kakuta on 12 August.

After spending the next few months working up, Hiyō arrived at Truk, together with her sister Jun'yō, on 9 October, to begin operations against American forces in the Guadalcanal area as part of the 3rd Fleet.

One of Jun'yō's B5Ns was forced to turn back with mechanical problems; the rest reached their objective and discovered two destroyers bombarding Japanese supply dumps on Guadalcanal around 07:20.

Hiyō's aircraft attacked USS Aaron Ward seven minutes later without effect, and the American ship shot down one B5N and damaged another which was forced to make a crash landing.

[14][15] A fire in the ship's generator room occurred on 21 October and reduced her top speed to 16 knots (30 km/h; 18 mph), so Kakuta transferred his flag to Jun'yō while Hiyō returned to Truk for repairs.

The remaining aircraft of her air group (16 Zeros and 17 D3As) were flown off for Rabaul, on the island of New Britain, on 23 October, from where the fighters escorted bombers attacking Guadalcanal the following day.

Escorted by 18 Zeros from Hiyō and the 204th Naval Air Group, 9 D3As slightly damaged three cargo ships in exchange for 4 dive bombers shot down and another forced to crash land.

[14] Her air group consisted of 27 Zeros and 12 D3As, and they were detached from Hiyō in early April to participate in Operation I-Go, a land-based aerial offensive against Allied bases in the Solomon Islands and New Guinea.

The following day, 17 of Hiyō's Zeros provided top cover for several waves of attacks on Port Moresby, New Guinea, her pilots claiming nine victories without losing any of their own.

On 14 April, the Japanese attacked Milne Bay, New Guinea, with a large force escorted by 75 Zeros contributed by all the carriers involved.

[19] In response to the invasion of Attu Island on 11 May, the Second Carrier Division departed Truk, accompanied by three battleships and two heavy cruisers, and reached Japan on 25 May.

Now the flagship of the Second Carrier Division under Rear Admiral Munetaka Sakamaki, Hiyō departed Yokosuka on 7 June with Junyō en route for Truk.

[22] The new base was closer to the oil wells in Borneo on which the Navy relied for fuel and also to the Palau and western Caroline Islands, where the Japanese expected the next American attack.

The location lacked an airfield on which to train the green pilots, and American submarines were very active in the vicinity which restricted the ships to the anchorage.

Ozawa decided to launch his air strikes early the following morning, so the Japanese turned south to maintain a constant distance between them and the American carriers.

A pair of Zeros and six D4Ys bound for Rota spotted the carriers Wasp and Bunker Hill en route, but failed to inflict any damage on them while losing five D4Ys to anti-aircraft fire.

They discovered the retiring Japanese fleet during the afternoon of the following day, and Vice-Admiral Marc Mitscher ordered an air strike.