These ships were s built by Mitsubishi at their Nagasaki shipyard for the shipping lines "Nippon Yusen Kaisha" (NKK) and "Osaka Shosen Kaisha" (OSK). "Nitta Maru" and "Yawata Maru" were ordered for NKK and both were completed before the beginning of the Pacific War in December 1941. The ships were intended for service to Europe, but the start of World War II in September 1939 restricted them to the Pacific. "Kasuga Maru" had been ordered by OSK and was fitting out when she was acquired by the IJN in 1940 and towed to Sasebo Naval Arsenal on 1 May 1941 to finish her conversion into an escort carrier. She was the first ship to be completed as her sister ships were not converted until 1942.