Akagi and Amagi were originally supposed to be completed as Amagi class battlecruisers, while Kaga (and Tosa) were supposed to be completed as Tosa class battleships. However, due to the naval treaties (can't recall whether it was the London or Washington one) which limited the tonnage of ships allowed in most navies, the Japanese had to scrap those plans. However, as a concession, they were allowed to choose two of their ships that were being constructed to be converted to aircraft carriers. Originally, Amagi and Akagi were chosen for conversion, with Kaga being either scrapped or used for target practice. However, the Great Kanto Earthquake wrecked Amagi's hull so badly that it was decided she would be scrapped instead and Kaga would take her place as a CV.