Sunfish was laid down on 22 July 1935 and launched by Lady Skipwith on 30 September 1936. Following the launch ceremony, she moved from the river into the basin via the north lock at 1255.

She was ordered under the 1934 - 1935 building programme. Sunfish differed from others in her class in that she had eight cylinder diesels instead of six.
The second vessel to have the name Sunfish, the first was in 1896.

On completion of workup, Sunfish joined the flotilla at Portsmouth and remained there until March 1939 when she returned to Chatham for a refit.
Sunfish was re-commissioned on 25 September 1939 and joined the 3rd flotilla at Harwich.
In late November 1939 she sailed with another Chatham boat, Snapper to search for the Scharnhorst.
Up until 9th April 1940, British submarines could only attack warships and transports without warning. On this date the order was given to attack all German shipping. Five hours after the order was received, Sunfish attacked and sank the 7000 ton Amasis followed by the 2600 ton Antares the following day. On 13th April 1940, Sunfish damaged another ship which ran itself aground and then on the 14th, she sank the 6000 ton Florida.
On 9th May 1940 the commanding Officer, Lt Commander Slaughter received the DSO. In the same month the flotilla moved to Rosyth.
The next success for Sunfish came on 2nd December 1940. Whilst operating off Stattlandet, she sank the 2200 ton Oscar Midling. Five days latter she attacked and damaged a tanker.
In January 1941 on Norwegian coast patrol Sunfish landed agents. In February she attacked without success a merchant ship and was then counter attacked but received no damage.
From August 1942 until June 1943 Sunfish was in refit. Following the refit, Sunfish spent 1943 and 1944 on training duties.
In 1944 four boats were transferred to the Russian Navy. Of these Sunfish was one and she was renamed B1.
Having sailed from Lerwick for Murmansk in July 1944, B1 was mistakenly attacked and sunk by the RAF.