Seaplane carriers were part of the Imperial Japanese Navy's plan heading into the post-World War 1 period and this led to a commitment of two such vessels, IJN Shokaku and IJN Hosho for the role. The former was cancelled and the latter was eventually evolved from its original seaplane form to a dedicated aircraft carrier in 1921 for the IJN after Japanese observers in Britain liked what they saw with HMS Hermes. The hulk of a naval oiler (laid down in 1919) was used in her construction but she was to be a purpose-built vessel - the second of its kind in the world after HMS Hermes - though IJN Hosho beat Hermes to the finish line by becoming the world's first commissioned aircraft carrier.
The shift in priorities from seaplane carrier to aircraft carrier forced a reworking of the propulsion scheme from triple-expansion steam-based engines to a pair of military marine-grade geared steam turbines to help fulfill the speed requirement of 25 knots. Fed by eight Kampon Type B boiler units, horsepower output reached 30,000 and two shafts were in play under stern. A flush, flat top flight deck running from bow to stern forced exhaust systems to be set along the starboard side. These funnels were further hinged so as to fold down when aircraft would be operating on deck. Hangar elevators allowed movement of warplanes from below deck to the flight deck. A navigation bridge superstructure was added for command and control though this structure was eventually deleted in 1923. Twenty-one aircraft could be carried of the 32 originally planned but, in practical use, the warship settled for fifteen. The crew complement numbered 512.
Finalized dimensions included an overall length of 552 feet, a beam of 59 feet and a draught of 20.2 feet. Displacement was 7,600 tons under standard load. Operational ranges peaked at 8,700 nautical miles.
For point defense, the vessel was outfitted with 4 x 140mm /50 caliber 3rd Year Type Anti-Aircraft (AA) guns and 2 x 80mm /40 caliber 3rd Year Type AA guns.
IJN Hosho was launched on November 13th, 1921 and entered service through commissioning on December 27th of that year. Her early career saw her used as a training and testing platform for a new generation of Japanese naval aviators and warplanners. By 1924, any outlying structures hampering efficient deck operations were stripped.
The warship's career saw it participate in the "Shanghai Incident" of 1932 against China and, later, in the Sino-Japanese War (1937-1945) proper. These two campaigns provided a wealth of knowledge and experience to the IJN and aided in its future offensives across the Pacific during World War 2 (1939-1945) though the vessel's fifteen-strong air arm limited its tactical and strategic value for the long term. From 1939, IJN Hosho was placed in reserve and used for additional training initiatives.
All that changed when the United States went to war with Japan. The vessel made up part of the attacking force against Pearl Harbor, Hawaii (December 7th, 1941) and was present during the Battle of Midway (June 4th - 7th, 1942) though not in a direct, frontline role. The assault on the island proved devastating for the IJN as four carriers were lost in the action. Short on carriers, the service commissioned other vessels to help stem the tide but Hosho was sent to reserve role once again to train more airmen and support personnel from 1942 onward.
Amazingly for an IJN wartime vessel, Hosho managed to survive all of the war, suffering damage from running aground in 1944 and from American aerial bombs. She was allowed to be used as a transport for Japanese military personnel in the immediate post-war period. Tens of thousands of men were brought back to the Japanese mainland by Hosho (she was further modified for the role in December 1945). In early 1946 into 1947, she was stripped of her usefulness and her hulk scrapped. Her name was officially struck from the Naval Register on August 31st, 1946.
Hosho was critical to the design of several key Japanese carriers of the wartime period - namely Akagi, Kaga and Ryujo.