[10][Note 1] In the lead-up to the Pacific War the Imperial Japanese Navy (IJN) attempted to build a large number of fleet carriers.

For them to be built quickly, the design for these ships was based on the aircraft carrier Hiryū rather than the newer and more sophisticated Taihō or the Shōkaku class.

Eventually, only three (Unryū, Amagi, and Katsuragi) were completed and construction of the other three carriers (Kasagi, Aso and Ikoma) was abandoned in 1945.

The IJN unofficial designation for Unryū and Amagi were Modified Hiryū class (改飛龍型, Kai Hiryū-gata),[13] Ship Number 5002–5006 were Modified Unryū class (改雲龍型, Kai Unryū-gata)[14] also.

[18] The IJN unofficial designation for this class was Modified Ship Number 302-class (改第302号艦型, Kai Dai 302-Gōkan-gata).