The submarines of the K6 sub-class were versions of the preceding K5 sub-class with greater range and diving depth. They displaced 11150NaN0 surfaced and 14470NaN0 submerged. The submarines were 80.5m (264.1feet) long, had a beam of 7m (23feet) and a draft of 4.07m (13.35feet). They had a diving depth of 80m (260feet).[1]
For surface running, the boats were powered by two 21000NaN0 diesel engines, each driving one propeller shaft. When submerged each propeller was driven by a 600hp electric motor.[2] They could reach 19.75kn on the surface and underwater. On the surface, the K6s had a range of 11000nmi at ; submerged, they had a range of 45nmi at .
The boats were armed with four internal bow 53.3cm (21inches) torpedo tubes and carried a total of ten torpedoes. They were also armed with a single 76.2mm L/40 anti-aircraft gun and two single 251NaN1 AA guns.[3]
Ro-40 was laid down on 8 August 1942 by Mitsubishi at Kobe, Japan, with the name Submarine No. 206.[4] She was renamed Ro-40 on 5 February 1943 and was attached provisionally to the Maizuru Naval District that day.[4] She was launched on 6 March 1943[4] and completed and commissioned on 28 September 1943.[4]
Upon commissioning, Ro-40 was attached formally to the Maizuru Naval District and assigned to Submarine Squadron 11 for workups.[4] During a training cruise, she collided with the sailing vessel Okaki Maru in the Seto Inland Sea off Murozumi Lighthouse on 5 October 1943, with both ships suffering minor damage.[4] In late November 1943, she took part in antisubmarine warfare exercises in the Iyo Nada in the Seto Inland Sea with the submarine tender and the submarines,,, and .[5] She called at Tokuyama to refuel from 2 to 4 December 1943.[4]
As of 1 January 1944, Ro-40 was assigned to Submarine Division 11 in Submarine Squadron 7 along with I-42, I-43, I-184, Ro-113, and the submarines,,,, and .[4] On 15 January 1944, she was reassigned to Submarine Division 34.[4] She departed Maizuru on 20 January 1944 bound for Truk, which she reached on 29 January 1944.[4]
Ro-40 got underway from Truk on 12 February 1944 to begin her first war patrol, ordered to operate in the Marshall Islands and then proceed to a patrol area east of the Gilbert Islands in the vicinity of Makin Island.[4] The Japanese never heard from her again.[4]
On 16 February 1944, the United States Navy destroyer was northwest of Kwajalein when she made sonar contact at a range of 1700yd on a submerged submarine approaching the convoy she was screening.[4] After Phelps dropped a pattern of 13 depth charges, the destroyer and minesweeper also depth-charged the submarine, sinking it at 9.8333°N 201°W.[4]
The submarine Phelps, MacDonough, and Sage sank probably was Ro-40.[4] The commander-in-chief of the 6th Fleet, Vice Admiral Takeo Takagi, ordered her to a new patrol area between Kwajalein and Eniwetok on 20 February 1944[4] and ordered her to return to Truk on 4 March 1944, but she did not acknowledge either order.[4] On 28 March 1944, the Imperial Japanese Navy declared her to be presumed lost in the Gilbert Islands area with all 61 hands.[4] She was stricken from the Navy list on 30 April 1944.[4]