It should be noted that Kaga was originally slated to be a battleship. However, the Washington Naval caused the IJN to stop construction and convert Kaga to an Aircraft Carrier.

Source: http://en.wikipedia.org/wiki/Tosa-class_battleship#Cancellation_and_fates

That and the Great Kanto Earthquake damaging Akagi's sister ship Amagi beyond repair. Kaga was the only other hull on hand that was sufficiently large to convert into a carrier, so Amagi's misfortune saved her from the scrapyard.