Keiichi Arima enrolled in the Imperial Japanese Naval Academy in April 1933 and graduated from 64th class in March 1937. He received his commission as an Ensign in March 1938. In August of the same year he was sent for an aerial training program to Kasumigaura Training Naval Air Station near Tokyo and completed it in March 1939. He then was chosen for advanced aerial training courses at Yokosuka NAS near Tokyo and Saeki NAS on Kyushu, where he specialized in aerial reconnaissance. He was promoted to Lieutenant Junior Grade in June 1939. He was assigned as a junior flight division leader to the 12th Air Group, which was stationed in Central China. His unit flew Aichi D1A dive bombers out of the bases in Anqing, Jiujiang, and Hankou to support ground operations in the area. Later on, the unit switched to Aichi D3A dive bombers and participated in Bombing of Chongqing, which was the capital of the Chinese Nationalists at that time. He was promoted to full Lieutenant in May 1941. Lieutenant Arima was transferred to the carrier Sōryū in the end of 1940 and was assigned as a junior division leader of dive-bomber squadron. After six months, he was reassigned to Suzuka NAS in Mie Prefecture, where he served as an instructor when Empire of Japan entered the war with United States of America.
Pacific War
In May 1942, Lieutenant Arima was transferred to the carrier Shōkaku and was assigned as a senior flight division leader of dive-bomber squadron. In late August 1942 Shōkaku participated in the Battle of the Eastern Solomons. During the attack on the US carriers, Lieutenant Arima was part of Lieutenant Commander Mamoru Seki's first strike wave, consisting of 27 D3A dive bombers and 10A6M Zero fighters. Arima led one of the three divisions, with nine dive bombers under his command. He was in the observer seat of D3A, while his aircraft was piloted by Petty Officer First Class Kiyoto Furuta. The USN Combat Air Patrol of Grumman F4F Wildcat fighters intercepted the formation of dive bombers, however Arima's division managed to escape into cumulonimbus clouds and approached the carrier Enterprise unharmed. The division then made a dive-bombing attack on Enterprise and released their bombs around an altitude of 500 meters. They scored three hits, with Arima's being the first. His 250 kilogram semi-AP bomb penetrated the starboard forward corner of her number 3 elevator aft and sliced through to the third deck before detonating in the chief's quarters. After the attack, Arima remained north-west of the burning Enterprise to wait for the friendly Zero fighters in order to led them back to the Japanese carriers. However, he encountered several US aircraft that started to case his D3A. He and his pilot evaded the attackers by flying very close to the sea surface and eventually made it back to Shōkaku after sunset. In late October 1942, Shōkaku was again dispatched to Solomon Islands to support the Imperial Japanese Army ground assault on Henderson Field on Guadalcanal. During the Battle of the Santa Cruz Islands, Lieutenant Arima's division was part of the second strike wave that consisted of 27 D3A dive bombers and 5 A6M Zero fighters, again commanded by Lieutenant Commander Seki. The Zero fighters were led by Lieutenant Hideki Shingō, the fighter squadron leader of Shōkaku. Arima and his pilot attacked Enterprise from astern and released the bomb at an altitude of between 450 and 500 meters and scored a hit. Their 250 kilogram semi-AP bomb penetrated the center of the flight deck, around six meters from the forward edge, and detonated inside the carrier, causing fires. They did not encounter any CAP on the way out and they climbed to 6000 meters, where Arima observed a dense smoke coming from Enterprise. Despite the successful attack, it came at a heavy price, as Lieutenant Commander Seki was killed. As a simultaneous USN strike damaged the flight deck of Shōkaku, leaving her unable to launch or recover aircraft, Arima's D3A landed on Zuikaku.
Later career
After the Battle of the Santa Cruz Islands, Lieutenant Arima was transferred to Usa NAS in Ōita Prefecture on Kyushu and served as an instructor. He was then transferred to Yokosuka NAS, where he served as a flight commander. There he was involved in developing communication equipment and radar, and provided instructions on the developed hardware. In October 1944, he was promoted to Lieutenant Commander. He survived the war.
