The Type C submarines were derived from the earlier KD6 sub-class of the Kaidai class with a heavier torpedo armament for long-range attacks.

[3] Ordered under the 3rd Naval Armaments Supplement Programme and built by Mitsubishi at Kobe, Japan, I-20 was laid down on 16 November 1937 with the name Submarine No.

[4] At 02:15 on 19 November 1941, the five submarines got underway from Kamegakubi bound for the Hawaiian Islands,[4] taking a direct route that took them south of Midway Atoll.

17′s conning tower in the wake of the general stores ship USS Antares (AKS-3), which was approaching the harbor′s outer gate with a target barge in tow.

[4] A PBY Catalina flying boat of Patrol Squadron 14 (VP-14) dropped smoke markers to indicate the midget submarine′s position.

[4] Ward opened gunfire on the submarine at 06:45 at a range of only 100 yards (91 m), firing the first shot of World War II by the American armed forces.

[4] On 28 August 2002, the Hawaii Undersea Research Laboratory deep submergence vehicles Pisces IV and Pisces V found a midget submarine resting almost upright on the bottom in 1,200 feet (366 m) of water 3 to 4 nautical miles (5.6 to 7.4 km; 3.5 to 4.6 mi) off the entrance to Pearl Harbor, prompting speculation among historians and maritime archaeologists that it was I-20′s midget.

[4] She surfaced 15,000 yards (13,700 m) off the harbor at Pago-Pago on Tutuila in American Samoa before dawn on 11 January 1942 and fired twelve 140-millimeter (5.5 in) rounds from her deck gun at the naval station.

[4] Neither vessel suffered damage, and Monowai transmitted a submarine contact signal and steamed out of the area at high speed.

[4] That morning, the commander of the 6th Fleet, Vice Admiral Teruhisa Komatsu, the commander of Submarine Squadron 8, their staffs, and the midget submarine crews paid a courtesy call on the commander-in-chief of the Combined Fleet, Admiral Isoroku Yamamoto, aboard his flagship, the battleship Yamato, at Hashirajima anchorage.

[4] During the detachment′s voyage, 16 United States Army Air Forces B-25 Mitchell bombers launched by the aircraft carrier USS Hornet (CV-8) struck targets on Honshu in the Doolittle Raid on 18 April 1942.

[7] The detachment received orders from the 6th Fleet that day to divert from its voyage and head northeast, passing north of the Bonin Islands, to intercept the U.S. Navy task force that had launched the strike.

[8] The rest of the "A" Detachment reached Penang on 27 April 1942, where the seaplane carrier Nisshin — which had undergone modifications allowing her to carry Type A midget submarines — rendezvoused with it.

[4] I-20 and the other "A" detachment units got underway from Penang on 30 April 1942, headed westward into the Indian Ocean with I-10 serving as the detachment′s flagship.

[4] I-20 suffered a mishap on 17 May when seawater entered through her main induction valve and flooded her engine room in heavy seas.

[4] I-10′s Yokosuka E14Y1 (Allied reporting name "Glen") floatplane began reconnaissance flights over ports in South Africa by reconnoitering Durban on 20 May 1942, followed by flights over East London, Port Elizabeth, and Simon's Town over the next week,[4] and by 24 May the "A" detachment submarines were encountering heavy Allied shipping traffic as they approached East Africa.

[4] On the night of 29 May, I-10′s floatplane flew over Diego-Suarez, Madagascar, sighting the British battleship HMS Ramillies among the ships anchored there.

[4] After the midget ran aground, its two-man crew reached shore and attempted to make their way overland to the designated recovery area, but they were reported to the British as having been seen outside Anjiabe at around 11:00 on 1 June and died in a gunfight with Royal Marine Commando No.

[4] Ramillies survived the attack and departed for Durban ten days later, although the Japanese assessed her as sunk after I-10′s floatplane noted her missing from Diego-Saurez during a reconnaissance flight.

[4] On 3 June 1942, after I-16 and I-18 had departed the recovery area, I-20 arrived and made an unsuccessful attempt to contact the midget submarines and their crews.

[4] On 21 July, she moved into the Gulf of Aden before departing her patrol area and proceeding to Penang, which she reached on 5 August 1942.

[4] Getting underway from Shortland for her second supply run, she arrived off Cape Esperance on 7 January 1943, discharging 18-tons of cargo in rubber containers.

[4] She unloaded 37 tons of cargo there and evacuated 39 men, including Imperial Japanese Army Lieutenant General Hatazō Adachi and his staff.

[4] On 30 August 1943, she reported sighting an Allied force including an aircraft carrier and two battleships off Espiritu Santo.

[4][10] The first action took place on 1 September 1943, when the destroyer USS Wadsworth (DD-516), operating as part of a hunter-killer group, began a search for a reported Japanese submarine off Espiritu Santo at 10:55.

[10] Wadsworth continued to pursue the submarine, which maneuvered to create more underwater turbulence in an attempt to defeat Wadsworth′s sonar.

[10] A PBY Catalina flying boat reported debris and a 400-by-600-yard (370 by 550 m) oil slick that smelled like diesel fuel on the surface just south of the location of Wadsworth′s final attack.

[10] The second action occurred on 3 September 1943, when the destroyer USS Ellet (DD-398) conducted a sweep for a reported Japanese submarine off Espiritu Santo.

[4] She lost sonar contact at 20:59, and at dawn on 4 September 1943 a large oil slick and debris were sighted on the surface at 13°10′S 165°28′E / 13.167°S 165.467°E / -13.167; 165.467.

[4][10] On 18 November 1943, the Imperial Japanese Navy declared I-20 lost with her entire crew of 101 men off Espiritu Santo.