 "Hiryū" was one of two large carriers approved for construction under the 1931–32 Supplementary Program.Originally designed as the sister ship of "Sōryū", her design was enlarged and modified in light of the "Tomozuru" and Fourth Fleet Incidents in 1934–35 that revealed many IJN ships were top-heavy, unstable and structurally weak.Her forecastle was raised and her hull strengthened.