The Tupolev ANT-40, also known by its service name Tupolev SB ( – Skorostnoi Bombardirovschik – high speed bomber) and development co-name TsAGI-40, was a high speed twin-engined three-seat monoplane bomber, first flown in 1934. The Tupolev design was advanced but lacked refinement, much to the dismay of crews, maintenance personnel, and Stalin, who pointed out that "there are no trivialities in aviation".
Numerically the most important bomber in the world in the late 1930s, the SB was the first modern stressed skin aircraft produced in quantity in the Soviet Union and probably the most formidable bomber of the mid-1930s. It was produced in the Soviet Union and was also built under license in Czechoslovakia. Many versions saw extensive action in Spain, the Republic of China, Mongolia, Finland and at the beginning of World War II against Germany in 1941. It was also used in various duties in civil variants, as trainers and in many secondary roles. Successful in the Spanish Civil War because it outpaced most fighters present (composed mostly of biplanes), the aircraft was obsolete by 1941 as faster fighters (such as the Bf-109) had by then been introduced. By June 1941, 94 percent of bombers in the Red Army air force (Soviet Air Force (VVS) Red Army (RKKA) were SBs.
Development
In 1933 the Soviet Air Force ministry (UVVS) issued an outline requirement for a high-speed bomber. Work on this proposal at TsAGI began in January 1934. The SB was designed and developed in the Tupolev KB ("Design Bureau") by a team led by A. A. Arkhangelski. Two versions were planned, one with Wright Cyclone radial engines (ANT-40 RTs), and one with the Hispano-Suiza 12Y liquid-cooled V12 engines (ANT-40 IS). The skills gained in the design of the MI-3 and DI-8 aircraft were widely used. The first two prototypes were called ANT-40.1 and ANT-40.2. The Cyclone powered prototype flew first, on 7 October 1934, while the first Hispano-Suiza powered prototype (ANT-401), which featured a larger wing, flew on 30 December 1934, demonstrating superior performance.Gunston 1995, p.405.Air International January 1989, p. 46.Maslov 2007, p.64.
The second Hispano-Suiza powered aircraft, the ANT-402 was considered a production prototype, and its performance was impressive but it was plagued by teething problems, leading unhappy test personnel to cover the ANT-402 with placards listing the aircraft's defects prior to a visit by Sergo Ordzhonikidze, the Commissar for Heavy Industry. On seeing these placards, Ordzhonikidze summoned Tupolev to a meeting at the Kremlin to discuss these shortfalls. When Tupolev stated that most of the defects were trivial, Joseph Stalin said,
The first production aircraft, designated SB, rolled off the production line before the end of 1935, and before ANT-402 had completed its flight test programme.Air International January 1989, pp. 47, 49. 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.Duffy and Kandalov 1996, p.80.
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 Soviet Air Forces (VVS) squadrons were in the process of working up with the new bomber.Air International January 1989, p. 51. Giving excellent performance in the Spanish Civil War, it acquired the popular name "Katyusha" (Catherine).Maslov 2007, p. 75.
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 Škoda 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 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.Air International February 1989, pp. 100–101.Maslov 2007, p.78. Sixty 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 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.Air International February 1989, p. 81.Gunston 1995, p.406. 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.Air International February 1989, pp. 80–81.Maslov 2007, p.74.
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 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.Maslov 2007, pp. 67–70. 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 .Duffy and Kandalov 1996, p. 82.
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, SB-RK (later renamed 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.Air International March 1989, pp. 148–149.Maslov 2007, p. 71.
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.Air International March 1989, p. 153. 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.Duffy and Kandalov 1996, p. 222. 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".Jackson 2003 p. 150.
Operational history There were a number of foreign customers for the SB. They were mostly satisfied with the aircraft's performance. There were some complaints about the noise, cramped crew compartments, hard undercarriage suspension and in particular about the front gunner's position, which could be reached only through a hatch under the fuselage, preventing the gunner from escaping in the event of a ditching or belly landing. Czechoslovakia signed an agreement to produce the ANT-40 as Avia B-71.Jackson 2003 p. 151.
Spanish Civil War
While only 54 SBs had been delivered to the Soviet Air Forces by 1 July 1936,Maslov 2007, p. 67. this did not stop the new Tupolev bomber being amongst the first shipments of military equipment sent by the Soviet Union to support the Spanish Republicans when the Spanish Civil War broke out on 17 July 1936. An initial batch of 31 SBs arrived in Cartagena aboard the Soviet Freighter Komsomol in October 1936, flying their first mission, a bombing raid by four SBs against Tablada airfield, Seville on 28 October. The SBs were used to equip Grupo 12 of the Spanish Republican Air Force, which at first was mainly manned by Soviet volunteers and under Soviet control.Martinez 1986, pp. 45–47.
The SB could outpace the Fiat CR.32 and Heinkel He 51 biplane fighters of the nationalist forces, and was therefore difficult to intercept, with dives from high altitude being the only way to intercept the SB.Maslov 2007, p.76. On 29 May 1937 two SBs attacked the German pocket battleship Deutschland, mistaking it for the Nationalist cruiser Canarias, killing 31 and injuring an additional 83 German sailors.Martinez 1986, p. 49. In June–July, a second consignment of 31 SBs were received, allowing Grupo 12 to return to full strength, and a new unit, Grupo 24, to be established.Martinez 1986, p.51. The delivery of Messerschmitt Bf 109s to re-equip the German Condor Legion meant that the SB could no longer evade Nationalist fighters by sheer speed, and losses rose.
A third and final batch of 31 SBs arrived in June 1938,Air International March 1989, p. 151. allowing operations to continue, although losses continued to be high. By the time the Civil War ended in April 1939, 73 SBs had been lost, 40 of them to enemy action. Nineteen SBs were taken over by the Nationalists, and used to form a bomber squadron. Although some were re-engined with French Hispano-Suiza 12Ybrs engines to aid maintenance, they were still subject to spares shortages, and in April 1943 only three were airworthy. When Junkers Ju 88s were received in December 1943, the remaining SBs were used for occasional training flights until withdrawn and scrapped in 1948.
China
In July 1937, the Second Sino-Japanese War broke out. The Soviet Union signed the Sino-Soviet Non-Aggression Pact on 21 August 1937, and as part of this agreement, supplied large amounts of military equipment to the Chinese Nationalists, as well as deploying complete air force units, nominally manned by Soviet volunteers. An initial delivery of 62 SBs was made in September–October 1937, with combat operations by Soviet forces starting in December with attacks on Japanese ships on the Yangtze River. On 23 February 1938, to celebrate Soviet Army Day, Soviet SBs carried out a long range attack on Japanese airfields on Taiwan, claiming 40 Japanese aircraft destroyed on the ground.Air International February 1989, p. 101.Maslov 2007, pp. 77–78.