"Sōryū" was one of two large carriers approved for construction under the Imperial Japanese Navy's 1931–32 Supplementary Program (the other being her near-sister "Hiryū").In contrast to some earlier Japanese carriers, such as and , which were conversions of battlecruiser and battleship hulls respectively, "Sōryū" was designed from the keel up as an aircraft carrier and incorporated lessons learned from the light carrier .