Upon commissioning, "I-55" was based in the Kure Naval District and assigned to Submarine Squadron 11 in the 6th Fleet. In late June 1944, she was selected for conversion to carry Special Naval Weapon No. 8, a version of the Fu-Go balloon bomb that could be launched at sea, the conversion involving the installation of hydrogen and balloon-launching equipment. Meanwhile, however, the Combined Fleet had activated Operation A-Go for the defense of the Mariana Islands on 13 June 1944, and the Marianas campaign had begun with the U.S. invasion of Saipan on 15 June. Before her conversion could begin, "I-55" got underway from Kure on 30 June 1944, called at Yokosuka from 1 to 6 July 1944, and then departed for Guam towing an "Unpoto" gun container, a sled that could carry up to 15 tons of cargo, usually in the form of three Type 96 howitzers and ammunition for them.