I-25, of 2,369 tonnes (2,600 tons), was 108 m (354 ft) long, with a range of 25,928 km (14,000 nmi; 16,111 mi), a maximum surface speed of 43.5 km/h (23.5 kn; 27.0 mph) and a maximum submerged speed of 15 km/h (8 kn; 9 mph). She carried a two-seater Yokosuka E14Y reconnaissance floatplane, known to the Allies as "Glen". It was disassembled and stowed in a hangar in front of the conning tower.
In World War II, I-25 served under the command of Lieutenant Commander Meiji Tagami who had graduated from Class 51 at Etajima, Hiroshima. 26-year-old Lieutenant Tatsuo Tsukudo was the Executive Officer(XO) on I-25. I-25 departed Yokosuka on 21 November 1941 in preparation for hostilities.
I-25 and three other submarines patrolled a line 222 km (120 nmi; 138 mi) north of Oahu during the Japanese attack on Pearl Harbor. After the Japanese aircraft carriers sailed west following the attack, I-25 and eight other submarines sailed eastwards to patrol the west coast of the United States. I-25 patrolled off the mouth of the Columbia River. A scheduled shelling of American coastal cities on Christmas eve of 1941 was canceled because of the frequency of coastal air and surface patrols.
I-25 attacked SS Connecticut 16 km (9 nmi; 10 mi) off the US coast. The damaged tanker managed to escape but ran aground at the mouth of the Columbia River. I-25 then returned to Kwajalein, arriving on 11 January 1942 to refuel and be refurbished.
I-25 left Kwajalein atoll in the Marshall Islands on 5 February for its next operational patrol in the south Pacific. Tagami's orders were to reconnoitre the Australian harbours of Sydney, Melbourne and Hobart followed by the New Zealand harbours of Wellington and Auckland.