(US Naval Historical Center photo, scanned from Polmar & Carpenter's "Submarines of the Imperial Japanese Navy")

These boats, based on the KD2 and U-139 designs, were of a junsen, or cruiser, type.  Design endurance was 60 days and their range was an impressive 24,000 miles.  Their MAN diesel engines, purchased from Germany, gave them a modest surface speed of 18 knots, although some of these boats exceeded 19 knots on trials.  Already elderly by 1941, they saw some combat in the war, but were among the first Japanese submarines converted to supply duty.

I-1 started the war off Hawaii, and shelled the harbor at Hilo.  Later, she patrolled the East Indies, then witnessed the Doolittle raid on Tokyo, before patrolling the Aleutians.  Adapted to a cargo role, her after 14cm gun was removed to make room for a 46-foot daihatsu barge.  On a supply mission to Guadalcanal, she was attacked by the New Zealand frigates Kiwi and Moa and, after a 90-minute battle, sank just  off the beach on 29 January 1943.  The crew took the current code books ashore, but left past and future codes aboard.  Her bow remained out of the water, so the Japanese tried to destroy the boat by demolition, submarine torpedo, and air attack, but all attempts failed.  The US Navy salvaged remaining code books, charts, manuals, and the ship's log, a total of 200,000 pages of intelligence booty.

Through radio intelligence, I-3 was ambushed off Guadalcanal by a pair of PT boats on 10 December 1942.  I-4 was torpedoed off that same island by USS Seadragon 10 days later.  I-2 survived until 7 April 1944 when she was sunk by destroyer USS Saufley off New Ireland.