The Letov Š-31 was a fighter aircraft produced in Czechoslovakia in the early 1930s in a number of variants. All of the aircraft had metal tubular framing and fabric covering with a metal engine cowling.
The first flight of the definitive and highly altered Š.231 version was on March 17, 1933. After testing at the Czechoslovak flight facility at Prague-Lethany, modifications were undertaken to improve the machine’s performance. It entered production the following year and began equipping Czech fighter units in June 1936. The machines however did not remain in front line fighter status with the Czechoslovak Air Force until the German occupation of Czechoslovakia in 1939. The sole Š.331 and 22 out of 24 produced Š.231s were sold to representatives of the Spanish Republican government. Reports of their combat record in Spanish Civil War are vague, but at least three machines survived the war and were used by victorious Nationalists. The operational performance and ultimate fate of the Š.331 is unrecorded.