A baptism of fire for the SB bombers occurred in the autumn of 1936, in the course of the Spanish Civil War. On October 23, 1936, the MS Stariy Bolshevik delivered the first SB 2M-100 bombers to the Spanish port of Cartagena. They were accompanied into Spain by Soviet pilots and maintenance personnel. These aircraft were nicknamed "Katyushka" ("Little Kate") in the Republican Air Force.
The first SBs supplied by the Soviet Union were equipped with old 7.62 mm Degtyaryov DA magazine-fed machine guns, instead of the quick-firing ShKAS machine guns, which were considered a secret weapon.
Combat experience showed that this solution was unsuccessful. The DA machine guns proved completely unfit for high-speed aircraft. The airflow generated resistance for the machine gun's return spring, and during high-speed flights the automatic equipment failed.
When Katyushkas appeared in Spain, the effect was stunning. The SBs could operate without any fighter cover. The bomber crews practically ignored Nationalist He 51 and CR.32 biplane fighters. M-100 engines with superchargers mounted on the aircraft had better altitude characteristics than those fighters' engines. The fighters simply could not overtake the SBs at a height over 4,000 m. Sometimes it was the Katyushkas that attacked and scattered the Francoist insurgents' bomber units.
As combat operations in Spain continued, work was done to improve the aircraft's bomb armament. A system of horizontal bomb racks, which was more advantageous in terms of ballistics, was created, to suspend bombs weighing 50, 70, and 100 kg. As a result, the plane's bomb capacity increased to 800 kg. Bomb racks from the Polikarpov R-Z biplane were used.
Spain saw the development and use of bomb clusters that allowed an SB to carry 48 high-explosive 12 kg bombs or 436 incendiary 0.76 kg bombs.
Many Spanish SBs had the sliding canopy of the gunner's rear cockpit removed. A small transparent windshield was installed to shield the gunner from the air flow, and a rear-view mirror was fitted in the upper section of the pilot's cockpit canopy.
A total of 92 SB bombers were supplied from the Soviet Union to Spain. The last batch, consisting of SB 2M-100As featuring more powerful M-100A engines, was delivered in March 1938.
The Republican aviation forces lost 74 of the 92 delivered SBs in combat operations. After the fall of the Spanish Republic in March 1939, the Nationalists obtained 19 operable Katyushkas. These planes, designated 20W (later changed to B.5), were flown in the Spanish Air Force. Their operation continued until 1946.