more powerful M-103. While the engine installation of the SB 2-M103 initially retained the drag inducing frontal radiators of the M-100 powered aircraft, an improved engine installation was developed with the radiators slung under the engines. On 2 September 1937 M.Yu. Alexeev set an official altitude record of with load of in an M-103 powered SB. He had earlier set an unofficial record of . In an attempt to further improve the performance of the SB, which by 1939 was becoming obsolete, the development of two second-generation versions were authorised, a direct replacement for the SB and a specialised dive bomber. The level bomber, known as the SN-MN or MMN, had a new wing of reduced wing area and was powered by more powerful Klimov M-105 engines. Performance was little better than the standard aircraft and it was abandoned. The dive-bomber, designated SB-RK (later redesignated Arkhangelsky Ar-2 after its designer, with Tupolev having been imprisoned and being in disgrace) was similar to the MMN but was fitted with dive brakes and it was ordered into production. Even though the SB was no longer a state of the art aircraft, production continued to increase through 1939 and 1940, as the Soviet Union tried to build up the strength of their air forces to compete with the growing threat of Nazi Germany, with almost 4,000 being built in these two years. The SB was phased out of production in early 1941, being replaced by the Petlyakov Pe-2. A total of 5,695 were built at Factory No 22 at Moscow before it was evacuated to Kazan, while Factory No 125 built a further 1,136 at Irkutsk. Three prototypes were built at the Tupolev design bureau, while Aero Vodochody and Avia in Czechoslovakia built 45 and 66 respectively, giving a total of 6,945 built. Design The SB was an all-metal monoplane powered by two Klimov M-100 12-cylinder water-cooled engines (license production version of Hispano-Suiza 12-Yrds engine) which drove fixed-pitch two-bladed metal propellers. The engines were provided with honeycomb-type frontal radiators enclosed by vertical thermostat-controlled cooling shutters. At an early production stage, the M-100 engine gave place to an improved M-100A engine, driving ground-adjustable three-pitch propellers, with speed being boosted to at . Because of its broad, high aspect ratio wing, that gave it a good altitude performance – Soviet crews nicknamed the SB the "Pterodactyl". Operational history There were a number of foreign customers