All the other OKB design teams and TsAGI departments were also involved. A team under V. M. Petlyakov worked on the wings and aileron controls, A. K. Martynov was in charge of the aerodynamics and matters relating to structural strength were dealt with by A. I. Makarevskiy, S. N. Shishkin and G. A. Ozerov.
After three months of intense work the draft project for the new bomber was submitted to the UVVS for approval on 8th March 1934. It was to be twin-engined three-seat cantilever mid-wing monoplane with a retractable undercarriage and smoothskinned fuselage and wings. The layout took as its premise that, given a bomb load of 500 kg and strong defensive armament, the aircraft should be as small as possible. The entire bomb load was carried internally and the aerodynamic shape of the machine had no sharply angular features. Thus the ANT-40 project became the logical outcome in the search for the optimum development and resolution of the ideas underlying the design work on the ANT-21, ANT-29 and ANT-30 projects.
The mock-up was approved on 10th March and the decision taken to build a pair of prototypes: one with air-cooled 730-hp Wright Cyclone radials and the second with liquid-cooled 780-hp Hispano-Suiza 12Ybrs Vee-12 engines. Construction began on 25th April, and on 7th October 1934 test pilot K. K. Popov made the first flight on the radial-engined ANT-40, alias SB-2RTS ('Rayt-Tsiklon' - that is, with two Wright Cyclone engines). On its ninth flight, this first machine had an accident and was returned to the factory for repair. It was subsequently used for experimental work.
The standard for series production was the second prototype with Hispano-Suiza engines (aka SB-2IS, for 'Ispano-Sooiza') started on 15th May. By 30th December the aircraft was ready and was flown that same day by I. S. Zhoorov. Manufacturer's tests lasted until 21st January 1935. A top speed of 430 km/h at 4,000 m was recorded, which significantly exceeded the requirements set down. Simultaneous state/service trials of the second prototype were held between 8th February and 3rd March 1935 but were interrupted by aileron flutter. This was corrected within the space of a few days by fixing counter-balance weights on the ailerons the work being carried out by Academician Mstislav V. Keldysh. The trials were resumed in the summer of 1935 and completed in April 1936. After this, the dooblyor was sent to production Factory No.22 as a standard for the production series.
The matter of the new bomber's series production had been settled much earlier back in March 1934. The VVS had an urgent need for this type of aircraft so that from March 1935 (5th December 1934 according to some sources) onwards, even before the state trials had been concluded, a start was made in providing Factory No.22 with the documentation. On government instructions the OKB's design team No.5 headed by Arkhangel'skiy was transferred to the production factory in the summer of 1936 as an independent experimental design bureau to facilitate the setting up of production there and the further development of the ANT-40 (SB).
The first production example, now fitted with 750-hp Soviet M-100 engines (hence the designation of the early production model, SB-2M-100), was rolled out in the spring of 1936. VVS trials of the first five experimental series examples took place between 25th March and 31st July 1936. It was on the results of these trials that the aircraft was accepted for service.
Mass production of the SB began at Factory No.22 in 1936, and the following year this was extended to Factory No.125 in Irkutsk. The mass production of an all-metal aircraft like the SB became a milestone in the annals of both Soviet and world aircraft design. Ten to thirteen examples were produced every twenty-four hours, and when production was finally halted in 1941 a total of 6,831 examples in various versions had been built.
The SB was modified several times in the course of production. Operational and combat experience was constantly taken into account in improving the aircraft and bringing its performance up to the necessary standard. Details of the versions which served with the VVS and GVF, as well as experimental modifications, appear below:
SB-2M-100A. The production SB with 860-hp M-100A engines of 1937;
SB-2M-100A with armour-protected gun positions. An experimental example drawing on the experience derived from using the first SBs in Spain underwent trials in 1937. Until 1941, however, only a handful such machines with Mozharovskiy/Venevidov MV-2 and MV-3 dorsal 'ball turrets' were manufactured;. Only after the Winter War with Finland (1939-40) were machines already in service equipped with these;
SB-2M-100A 'cruiser'. Two examples with a battery of four ShKAS machine-guns mounted in the nose underwent factory and range firing tests. They took part in the air defence of Moscow as night fighters in 1941;
SB-2M-100A with six RO-132 rocket launch rails. Six production examples were converted to carry six 132-mm RS-132 unguided rockets during the Winter War;
SB bis. An experimental example with 960 hp M-103 engines, nose radiators and three-blade VISh-2 variable-pitch propellers was tested in 1937-38;
SB bis-2. An experimental example with Klimov M-103 engines and chin-mounted radiators. It became the standard for series production;
SB bis-3. An experimental model with M-103A engines and ducted radiators. It became the standard for series production;
SB-2M-103. Version with chin radiators, strengthened airframe, external bomb racks and dorsal armour. Built in series 1938-39;
SB-2M-103 with ducted radiators. Production version and fastest of the SB series (top speed 450 km/h at 4,100 m). Built in series 1939-1940;
SB-2M-103 experimental dive-bomber conversion;
SB-2M-103 (SN). Experimental example with SN ultra-rapid-firing machine-guns;
SB-2M-103 'searchlight'. A production example fitted with a searchlight (in a similar manner to the Douglas A-20 Havoc/Turbinlite) to illuminate enemy bombers and used by Moscow's air defence forces during the Great Patriotic War;
SB-2M-103 'trawler'. A series example equipped as a flying 'trawler' (ie fitted with a grapnel on a cable and used to pull down enemy telephone lines);
SB-N. An SB-2M-103 with a ShVAK cannon mounted in the NU-SB nose gun position;
SB-UK (USB). Conversion trainer versions of the SB-2M-100A and SB-2M-103 with a second cockpit. Tested in the spring of 1938. Some service machines converted with a limited production series of SB-UK-2M-103s;
SB-2M-104. Small series of SBs with M-104 engines and revised armament;
MMN (maksimahl'naya modifikahtsiya izdeliya 'AT - 'maximum modification of 'product N', as the SB was coded in production). Comprehensive modification of SB with 1,050-hp Klimov M-105 engines, improved aerodynamics and augmented defensive armament. Passed trials in 1939 but was not series-built;
MMN-2M-104. Experimental second prototype of the MMN with M-104 engines;
SB-2M-105. Small series of SB with M-105 engines and new armament produced in 1940;
SB-2M-105 TK (TK-2). Three production SB-2M-105 fitted with turbosuperchargers (toorbokompressor). Not series-built;
SB-2M-100 floatplane. A project of 1934. Not built;
SB-2M-103 with a tricycle undercarriage. An experimental adaptation tested at the Flight Research Institute (Lll - Lyotno-issledovatel'skiy institoot) in 1940-41;
PS-40. Designation for demilitarised SB bombers passed down to GVF for use as cargo aircraft/mailplanes;
PS-41. Modernised PS-40 aircraft with M-103U engines. First conversion by GVF plant No.89, then other similar conversions of SB-2M-103 handed down from VVS;
PS-41 bis. Designation for a number of PS-41 fitted with additional underwing fuel tanks;
Avia B-71. Czech designation for the SB-2M-100A produced under licence from 1937 onwards. Examples were used by the air forces of Czechoslovakia, Germany, Bulgaria and Finland;
SB-RK (Ar-2). Comprehensive modification as a dive-bomber based on the MMN; the Ar prefix stood for A. A. Arkhangel'skiy. Placed in production shortly before the outbreak of the Great Patriotic War in 1941 and some 200 examples built.
The first SB bombers entered VVS service in 1936, and on 7th November that year formations of them flew over Red Square to commemorate the nineteenth anniversary of the Bolshevik Revolution. Even before that, in September, Soviet 'Volunteer-Internationalist' aircrews had flown in support of Republican Forces after the outbreak of the Spanish Civil War. The SB acquitted itself well in Spain and proved invulnerable for some time to attack by the slower German and Italian biplane fighters. This situation lasted until the debut of the Messerschmitt Bf109 in Spanish skies. Starting in the autumn of 1937, SB bombers flown by Soviet and Chinese aircrews took part in the Sino-Japanese conflict. The SB then saw action in the clashes with the Japanese at Lake Khasan and on the River Khalkhin-Gol and in the Winter War with Finland. By the outbreak of the Great Patriotic War in June 1941 the SB was already obsolescent, although it continued to form the backbone of the VVS's tactical bomber aviation and was used as a bomber on the Eastern Front until 1943. The major shortcomings of the SB as a combat aircraft were its inadequate defensive armament and almost complete absence of any armour protection for either the crew or the most vulnerable parts of the aircraft. These led to big losses inflicted by enemy fighters. After 1943 the SB remained in service as a transport, a tug for cargo gliders and gunnery practice targets, a training aircraft and a liaison aircraft.
A single example of the SB, restored through the efforts of the Tupolev OKB and its experimental factory, is currently preserved at the Central Russian Air Force Museum in Monino.
A number of world and national records were established by various versions of the SB. These include:
• On 1st November 1936 test pilot M. Yu. Alekseyev reached a height of 12,695 m with a 1,000 kg load on the second prototype ANT-40 (SB bis). This was registered as only a national record;
• On 2nd September 1937 Alekseyev set a world record by reaching a height of 12,246 m with a 1,000-kg load, flying an experimental SB bis-2.
Specifications of the SB-2M-100A
Length: 12.27 m
Wing span: 20.33 m
Height: 4.735 m
Wing area: 56.7 m²
Normal take-off weight: 5,732 kg
Bomb load: 500/600 kg
Maximum speed at 4,000 m: 423 km/h
Service ceiling: 9,560 m
Range: 1,500 km
Armament: 4 x ShKAS machine-guns