The S.XX was a refinement of the S.XVIII and was a two-seat fighter biplane which carried a pilot and tail gunner. The design was conventional but featured an upper wing with a pronounced sweep-back. This was joined to the lower wing by large I-struts. Like its predecessor, the S.XX became known colloquially as the "Herbemont", after its designer. Originally the French government issued an open-ended contract for these aircraft at the rate of 300 per month. However, this was cancelled at the Armistice, before any aircraft had been delivered. The order was later revived to obtain a modern fighter for France's post-war air force, and 95 were purchased. Additionally, the Japanese Mitsubishi company bought three examples, and the government of Bolivia bought one.
In 1918, a S.XX set the world airspeed record for a two-seat aircraft, with a speed of 230 km/h (143 mph), and in July 1919 Joseph Sadi-Lecointe set an altitude record of 8,900 m (29,200 ft) in one. This was the beginning of a prodigious streak of records set by a series of refined S.20s with increasingly shorter wingspans. Before the year was over, Sadi-Lecointe set two new speed records for the S.20's class: 249 km/h (155 mph) in September, and 252 km/h (157 mph) in October, as well as winning the Prix Henry Deutsch de la Meurthe competition five days earlier.
1920 saw S.20s used to break records not only in their own class, but the world absolute airspeed record three times over. On February 28, Jean Casale reached 283 km/h (176 mph), but this was soon broken twice by Bernard de Romanet, first on October 9 with a speed of 292 km/h (182 mph), and then on November 4 at 309 km/h (193 mph). The same year, two S.20s competed in the Gordon Bennett Cup. One, piloted by Casale, was disqualified, but the other, piloted by de Romanet placed second.
- S.XX - original military production version (95 built in series)
- S.XX bis - improved military version (2 built) with larger wings and tail. One sold to Mitsubishi
- S.20 bis-1 - racer
- S.20 bis-2 - racer with shortened wingspan
- S.20 bis-3 - racer with further shortened wingspan
- S.20 bis-4 - racer with even further shortened wingspan
- S.20 bis-5 - racer for Gordon Bennett cup (2 built). Radically modified wing design - upper wing changed to gull configuration.
- S.20 bis-6 - racer with shortened cabane struts and custom-built Hispano-Suiza engine