SB, rolled off the production line before the end of 1935, and before ANT-402 had completed its flight test programme. The aircraft entered full production in 1936, and was produced in two plants, State Aircraft Factory No 22 at Moscow and No 125 at Irkutsk until 1941. Despite the fact that the assembly lines were plagued with a constant string of modifications, some 400 SBs were delivered by the end of 1936—a number of these being diverted to Spain—and 24 VVS squadrons were in the process of working up with the new bomber. Giving excellent performance in the Spanish Civil War, it acquired the popular name "Katyusha" (Catherine). In 1937, negotiations were concluded between the Soviet and Czechoslovak governments, for the supply of SB bombers and a licence for local production, in exchange for the right to produce the Skoda 75 mm Model 1936 mountain gun. The version of the SB to be supplied to, and subsequently license-built as the Avia B-71 was the SB 2M-100A but fitted with the Avia-built Hispano-Suiza 12-Ydrs engine. A single 7.92 mm vz. 30 machine gun supplanted the twin ShKAS machine guns in the nose and similar weapons were provided for the dorsal and ventral stations. 60 aircraft were to be flown to Czechoslovakia by mid-1938. The planned licensed production program took a decidedly leisurely course, despite the increasingly dangerous political situation. By 15 March 1939, when the German Wehrmacht occupied Bohemia and Moravia, not one Czech-built aircraft had been delivered. Development of the SB continued, meanwhile, with revisions being made to reflect the lessons of early operations in Spain. As problems were encountered in converting pilots to fly the SB, a trainer version, the USB was built in September 1937, with a modified nose with an open cockpit for an instructor and dual controls. Problems were also encountered with the armament, the nose guns having limited traverse and so being little use against head-on attacks and later aircraft were modified with a better field of fire. From 1940, the dorsal gun position was replaced by an enclosed turret, while the ventral gun position, which was difficult to use, was also modified. The aircraft was also progressively fitted with improved engines. At first it was equipped with the Klimov M-100, a license-built version of the Hispano-Suiza 12Ybrs engine, but this was soon replaced by the more powerful M-100A, and from 1938 by the yet