[6] Compared to the German submarine, they were larger — 10 feet (3 m) longer, and displacing 220 more tons on the surface and 300 more tons submerged — and had a longer range both on the surface — 970 nautical miles (1,800 km; 1,120 mi) farther at 8 knots (15 km/h; 9.2 mph) — and submerged — 5 nautical miles (9.3 km; 5.8 mi) farther at 4.5 knots (8.3 km/h; 5.2 mph).

[2][3][4][5][6] Retaining their minelaying and torpedo capabilities, they were modified so that each of them could carry 15 tons of aviation gasoline with which to refuel flying boats,[2][3][4][5][6] allowing the flying boats to extend their range during reconnaissance and bombing missions by meeting the submarines in harbors and lagoons for more fuel.

[2][3][4][5] In the latter conflict, they laid mines and conducted anti-shipping patrols in East Asia and off Australia in the war′s opening weeks,[2][3][4][5] during which I-124 was sunk.

[5] The other three submarines supported Japanese operations during the Battle of Midway and the Guadalcanal campaign,[2][3][4] in which I-123 was lost.

[4] After service on supply runs during the New Guinea campaign, the two survivors, by then considered obsolescent, were withdrawn from combat in September 1943 and relegated to training duties in home waters,[2][3] during which I-122 was sunk in the last weeks of the war.