The Hiyo and Junyo started out as luxury liners with the Nippon Yusen Kaisha (N.Y.K.) line that were specifically designed to be later converted into aircraft carriers. They were designed with double keels, extra height between decks, storage for aviation fuel, and space for elevators. Conversion began in October 1940.
The Hiyo class was similar to the Soryu and Shokaku classes that preceded them.
There were two elevators in the flight deck, each about 45' x 45'. There were no catapults. The flight deck was made of wood planks.
There were two hangars in the Hiyo class. The hangers were only 10' 6" high which was severely cramped for aircraft.
The Hiyo class was the first to have an island that had the funnel on top that inclined outwards at 25° / 26°. As this was very successful it was copied in later carrier designs.