Ro-31 was a small Kaichu coastal submarine, a sister of the Ro-29 boats, built by Kawasaki at Kobe. She was laid down on 25 September 1921 and launched on 15 February 1923 as Submarine No. 70. She never finished that first life. On 21 August 1923, still on trials about two miles off Kariya Point on Awaji, she sank with a hatch open. Eighty-eight men were lost, yard hands among them, and five were saved. One account says a passing boat swamped her through that hatch. She was raised in the autumn of 1924. One date for the lift is 24 October, and another is November. The hull was taken apart and the material used again. She was laid down a second time on 20 December 1924, launched again on 25 September 1926, and completed on 10 May 1927. That day she was named Ro-31.
She spent the peacetime years at Sasebo. On 15 December 1938 she went into the reserve. The navy brought her back on 9 February 1942, for training in home waters. Mochitsura Hashimoto, just out of the submarine course, took her on 30 June 1942. He was a lieutenant then. On 1 November he was promoted to lieutenant commander, still in this boat. One list ends his tour in February 1943. The next captain both lists name is Motoo Nakamura, on 16 March 1943. No war patrol is recorded for her. The assignments after she recommissioned are the Kure and Yokosuka naval districts.
Nakamura’s relief is not a clean name. A one-month captain in June 1943 is printed in a form that does not agree with itself. Nobuo Kamimoto had her from 1 July 1943. On 31 January 1944 she went into the reserve at Kure and stayed there. She was struck on 25 May 1945. That is not the same thing as the reserve date. The hull was still in the western Inland Sea at the surrender, at Sasebo. American forces scuttled her off Sasebo on 5 April 1946. That is not the 1 April scuttling off the Goto Islands. She was not lost in a fight.