I-7 was an Imperial Japanese Navy J3 type submarine commissioned in 1937. She was a large cruiser submarine that served in World War II. She operated in support of the attack on Pearl Harbor, conducted anti-shipping patrols in the Indian Ocean, supported the Indian Ocean raid, and took part in the Guadalcanal campaign and the Aleutian Islands campaign. She was wrecked in the Aleutian Islands after a lengthy battle with the destroyer in June 1943.

Design, construction, and commissioning I-7 was the first of two Junsen III- (or "J3"-) type submarines. After the four Junsen I-type submarines (, , , and ), the Japanese had built as a modified Junsen I, introducing an aviation capability to the Junsen type with the inclusion of a hangar that allowed I-5 to carry and operate a floatplane. I-6, the only Junsen II-type submarine, represented the next step in the evolution of this aviation capability, as she had both a hangar and a catapult for a floatplane. The Japanese designed and equipped the next and last Junsen type submarines, I-7 and — the only two Junsen III-type submarines — to operate as submarine squadron flagships.Boyd and Yoshida, p. 22. In them, the Japanese sought to combine what they viewed as the best features of the earlier Junsen-type submarines with those of the Kaidai V-type submarines. Like I-6 before them, I-7 and I-8 had a hangar and catapult for a floatplane. They were the last Japanese submarines with aircraft facilities abaft the conning tower; all later Japanese submarines with aircraft capabilities had their hangars and catapults on their forward decks.Boyd and Yoshida, p. 23.

Built by the Kure Naval Arsenal at Kure, Japan, I-7 was laid down on 12 September 1934. She was launched on 3 July 1935 and was completed and commissioned on 31 March 1937.

Service history Pre-World War II Upon commissioning, I-7 was attached to the Yokosuka Naval District. On 1 December 1937, she became flagship of Submarine Squadron 1 in the 1st Fleet, a component of the Combined Fleet. She became flagship of Submarine Squadron 4 in the 1st Fleet on 15 November 1939, and on 11 October 1940 she was one of 98 Imperial Japanese Navy ships that gathered along with more than 500 aircraft on the Japanese coast at Yokohama Bay for an Imperial fleet review — the largest fleet review in Japanese history — in honor of the 2,600th anniversary of the enthronement of the Emperor Jimmu, Japan's legendary first emperor.

I-7 became flagship of Submarine Squadron 2 in the 6th Fleet, a component of the Combined Fleet, on 15 November 1940. While taking part in maneuvers in Saeki Bay on the morning of 21 October 1941, she collided with the submarine I-66. Both submarines suffered light damage.

On 10 November 1941 — by which time I-7 was serving in the 6th Fleet as flagship of Submarine Squadron 2 and was assigned to Submarine Division 8 with the submarines , , and . — the commander of the 6th Fleet, Vice Admiral Mitsumi Shimizu, gathered the commanding officers of the fleet's submarines together for a meeting aboard his flagship, the light cruiser , which was anchored in Saeki Bay. His chief of staff briefed them on the upcoming attack on Pearl Harbor, which would bring Japan and the United States into World War II. On 11 November 1941, I-7 was reassigned to the Submarine Advance Force.

As the Imperial Japanese Navy began to deploy on 16 November 1941 for the upcoming conflict in the Pacific, I-7 embarked both the commander of Submarine Squadron 2 and a Watanabe E9W1 (Allied reporting name "Slim") reconnaissance seaplane. At 13:00 that day, I-4, I-5, I-6, and I-7 got underway from Yokosuka bound for the Hawaiian Islands. While north of Oahu, I-7 received the message "Climb Mount Niitaka 1208" () from the Combined Fleet on 2 December 1941, indicating that war with the Allies would commence on 8 December 1941 Japan time, which was on 7 December 1941 on the other side of the International Date Line in Hawaii.

World War II First war patrol On 7 December 1941, the submarines of Submarine Squadron 2 took up patrol stations across a stretch of the Pacific Ocean from northeast to northwest of Oahu, with I-7 operating as their flagship. The submarines had orders to conduct reconnaissance in the area and attack any ships which sortied from Pearl Harbor during or after the attack, which occurred that morning. On 10 December 1941, Katori transmitted a message from Shimizu to the squadron commander ordering I-7′s floatplane to conduct a reconnaissance flight over Pearl Harbor to report on damage inflicted during the attack three days earlier and the progress the Americans were making on repairs. Accordingly, I-7 launched her floatplane early on the morning of 16 December 1941 from a position west of Kailua-Kona on the island of Hawaii. At 07:10, the floatplane's observer reported four battleships — one of them heavily damaged — and an aircraft carrier in Pearl Harbor off the East Loch anchorage and five cruisers and 30 smaller vessels including three destroyers in the harbor south of Ford Island. The floatplane returned to I-7 and landed next to her at 09:45 and, after its two-man crew abandoned the plane and swam to the submarine, I-7 scuttled the plane, submerged, and left the area.

On 17 December 1941, I-7 received orders to move to an operating area southwest of Oahu to support a sweep line by Submarine Squadron 2. Detached from that duty on 21 December 1941, she shifted to a patrol area southeast of Oahu. While southeast of Oahu on 1 January 1942, I-7 sighted a light cruiser and two destroyers steaming toward Pearl Harbor and fired two torpedoes at them, both of which missed. The destroyers counterattacked, dropping six depth charges, but I-7 escaped unscathed. On 9 January 1942, the submarine reported sighting the aircraft carrier northeast of Johnston Island, and I-7 diverted from her patrol to take part in the hunt for Lexington, but failed to find her. She then headed for Kwajalein, which she reached on 22 January 1942. She got back underway on 24 January 1942 bound for Yokosuka, which she reached on 2 February 1942.

Second war patrol While I-7 was at Yokosuka, Submarine Squadron 2 — consisting of I-1, I-2, I-3, I-4, I-5, I-6, and I-7 — was assigned to the Dutch East Indies Invasion Force in the Southeast Area Force on 8 February 1942, operating as Submarine Group C with I-7 designated the group's flagship. I-7 departed Yokosuka on 11 February 1942 bound for Palau, which she reached on 16 February, then got back underway on 17 February 1942 for the Netherlands East Indies. She arrived at Staring Bay on the Southeast Peninsula of Celebes just southeast of Kendari on 21 February 1942. At 06:00 local time on 23 February 1942, she began her second war patrol with the commander of Submarine Group C embarked, bound for a patrol area in the Indian Ocean south of Java. I-4, I-5, I-6, and I-7 formed a sweep line south of Java to interdict Allied shipping between Java and Australia, after which Submarine Group C patrolled south of the Cocos (Keeling) Islands.

At 12:30 on 2 March 1942, I-7 was operating in the Indian Ocean southwest of the Cocos Islands when an unidentified Japanese carrier aircraft attacked her by mistake. She suffered no damage. At 10:00 on 4 March 1942, she surfaced in the Indian Ocean northwest of the Cocos Islands and opened fire with her deck guns on the Dutch 865-ton motor vessel , a merchant ship on a voyage from Tjilatjap, Java, to Colombo, Ceylon, with a cargo of rubber. Merkus′s crew abandoned ship without loss of life, after which I-7 sank the ship with gunfire at . She concluded her patrol with her arrival at Penang in Japanese-occupied British Malaya on 8 March 1942.

Indian Ocean raid Orders arrived from the headquarters of the Combined Fleet for all the submarines of Submarine Squadron 2 except for I-1 to conduct reconnaissance operations along the coast of Ceylon and western coast of India in preparation for Operation C, the upcoming Indian Ocean raid by the aircraft carriers of the Combined Fleet's Mobile Force. Accordingly, I-7 departed Penang at 16:00 local time on 28 March 1942 with a Watanabe E9W1 (Allied reporting name "Slim") floatplane embarked. She was tasked to use the aircraft to conduct a reconnaissance of Colombo and Trincomalee, Ceylon, on 3 April 1942, two days before the scheduled Japanese carrier air strikes there.

At 05:17 on 1 April 1942, a Royal Air Force Catalina I flying boat, probably from No. 201 Squadron, attacked I-7 while she was on the surface in the Indian Ocean southeast of Ceylon. The Catalina dropped two bombs which landed close aboard, but I-7 suffered no damage. Four hours later she encountered several small Allied patrol vessels in the same area, and her commanding officer decided to cancel the reconnaissance flight scheduled for 3 April because of the enemy activity in the launch area. Instead, I-7 provided weather reports to the approaching Mobile Force.

At 03:40 on 3 April 1942, I-7 attacked the British 9,415-ton motor vessel in the Indian Ocean east of the Maldive Islands. I-7 fired two Type 89 torpedoes at Glenshiel, which was on a voyage from Bombay, India, to Fremantle, Australia, carrying 12 passengers and 1,000 tons of general cargo. One torpedo hit Glenshiel on her port side, and she began to settle by the stern. Glenshiel transmitted a distress signal and her crew and passengers — all of whom survived — abandoned ship. After waiting for the lifeboats to clear the ship, I-7 fired two more torpedoes, one of which hit. She then surfaced and opened fire on the ship with her deck guns, scoring 20 shell hits before the burning Glenshiel sank by the stern at .

Reassigned to the Advance Force on 10 April 1942, I-7 arrived at Singapore along with I-3 on 15 April 1942. She got back underway on 21 April 1942 and arrived at Yokosuka on 1 May 1942 to undergo repairs.

Third war patrol

While I-7 was at Yokosuka, the Aleutian Islands campaign began on 3–4 June 1942 with a Japanese air raid on Dutch Harbor, Alaska, followed quickly by the unopposed Japanese occupation in the Aleutian Islands of Attu on 5 June and Kiska on 7 June 1942. On 10 June 1942, I-1, I-2, I-3, I-4, I-5, I-6, and I-7 were reassigned to the Northern Force for duty in the Aleutians, and on 11 June 1942 I-7 set out for Aleutian waters to begin her third war patrol. She joined the "K" patrol line and patrolled off Unalaska Island. She was in the Gulf of Alaska on 14 July 1942 when she torpedoed the 2,722-ton United States Army Transport , which was on a voyage from Bethel, Alaska, to Seattle, Washington.wrecksite.eu SS Arcata (+1942) She then attacked Arcata with gunfire, her gun crew finding accuracy difficult in the choppy seas. After one of I-7′s shells hit Arcata′s bridge, her passengers and crew abandoned ship. Seven members of Arcata′s crew were killed. Japanese records claim I-7 ceased fire when she spotted life rafts in the water, while Arcata′s survivors reported that I-7 machine-gunned the life rafts, killing one crewman. Arcata sank at .

I-7 was assigned to the Advance Force on 20 July 1942. Her patrol ended with her arrival on 1 August 1942 at Yokosuka, where she began an overhaul.

Fourth war patrol