During the 1930s, Soviet designers produced a number of long-range aircraft which made several outstanding long-distance flights and drew attention to the capabilities of the Soviet air force. Of these aircraft, however, the Tupolev ANT-25 and ANT-37 were not successful in their military versions.
Sergei Vladimirovich Ilyushin was more successful in designing a long-range bomber able to carry a reasonable bomb load (one ton), fast (not less than 217 mph (350 kmh), at a reasonable height, and over a distance of 1,864 mls (3,000 km). Designated TsKB-26 (Central Design Bureau Number 26), the first prototype of the Ilyushin bomber made its first flight in 1935 with Vladimir Konstantinovich Kokkinaki at the controls, thus beginning a designer/test pilot combination which lasted for over thirty years.
The TsKB-26 was a twin-engined low-wing cantilever monoplane of metal construction with a retractable undercarriage, and was powered by two 800 hp Gnome-Rhone K-14 fourteen-cylinder radial air-cooled engines driving three-blade fixed-pitch metal propellers. Possibly because of the aircraft's large fuel capacity, there was only room in the internal bomb bay for 2,204 lb (1,000 kg) of bombs; but over short distances an additional 5,511 lb (2,500 kg) could be carried under the wing center-section. Defensive armament consisted of a 0.30 in (7.62 mm) machinegun in the nose and another in the hand-operated dorsal turret. The crew consisted of pilot, navigator/gunner and radio operator/gunner. Like most contemporary Gerrnan and American, and other Russian, medium bombers, the TsKB-26 had a comparatively narrow fuselage and a fighter-type cockpit which, in the case of the TsKB-26, was open.
The TsKB-26 under test had a range of 2,485 mls (4,000 km) with an 1,102 lb (500 kg) bomb load when flying at 199 mph at 15,090 ft (320 kmh at 4,600 m). It was extremely maneuverable, and, at the 1936 May Day parade over Red Square, Kokkinaki demonstrated this maneuverability by looping the TsKB-26. In July 1936 Kokkinaki set up two new international class records for altitude with payload, which he then proceeded to eclipse in August and September:
3 August, 1936 - altitude with 1,102 lb (500 kg) load, 43,234 ft (13,178 m)
21 August, 1936 - altitude with 2,204 lb (1,000 kg) load, 39,701 ft (12,101 m)
7 September, 1936 - altitude with 4,409 lb (2,000 kg) load, 36,095 ft (11,005 m).
The second prototype, designated TsKB-30, differed from the first by having an all-metal rear fuselage instead of wooden monocoque; and it was powered by the Soviet-built version of the Gnome-Rhone K-14, the M-85, which developed 760 hp for takeoff, 800 hp at 12,630 ft (3,850 m), and was enclosed in slightly cleaner nacelles. Variable-pitch propellers and spinners were fitted. The TsKB-30 also had a slightly modified fin and rudder and an enclosed pilot's cockpit. It entered production in this form in 1936 as the DB-3B, and the first deliveries to the air force were in 1937. Developments of the DB-3B, as the DB-3F and Il-4, remained in production until 1944, and throughout WW II the series was the backbone of the ADD (Aviatsiya Dalnevo Deistviya - Long Range Aviation).
Meanwhile, the TsKB-30 continued to break records. On 26 August, 1937, Kokkinaki, with A.M. Bryandinskii as navigator, flew Moscow-Sevastopol-Sverdlovsk-Moscow with a 2,204 lb (1,000 kg), and completed the 3,118 mls (5,018 km) at an average speed of 202 mph (325 kmh).
On June 27-28, 1938, the same pair flew in the red-painted TsKB-30, with the name Moskba (Moscow) stretching across the under surfaces of the wings, from Moscow to Spassk, near Vladivostok, a distance of 4,710 mls (7,580 km) in 24 hr 36 min nonstop, an average speed of 191 mph (307 kmh), and made mostly at 22,965 ft (7,000 m).
In 1939, the same aircraft, crewed by Kokkinaki and Major M.Kh. Gordienko, made a nonstop flight from the Soviet Union to the USA. Heavily laden with fuel, the Moskva took-off from Shchelkovno Aerodrome, Moscow, in the afternoon of 28 April. The flight proved to be an adventurous one, with strong winds, failure of the automatic pilot and freezing cold, which froze their sandwiches and eventually reached -65.2° F (-54° C) when they had to climb to 29,530 ft (9,000 m). By the time the Moskva was over the Gulf of St Lawrence, the fuel gauges showed empty and Kokkinaki was forced to look for an emergency landing ground. He found one in some marshland on the little island of Miscou at the northern tip of New Brunswick, and landed the Moskva with the undercarriage retracted. The aircraft was not badly damaged and the crew not seriously hurt. Kokkinaki and Gordienko had covered a distance of 4,971 mls (8,000 km) in 22 hr 56 min at an average speed of 348 kmh (216 mph) in appalling conditions. The airmen were taken off the island, and Soviet engineers arrived to collect the aircraft and ship it back to the USSR.
In the meantime, the Ilyushin KB was working on improving the DB-3 still further. In 1937, the M-85 engine was replaced by the M-86 which developed 950 hp for takeoff, and 800 hp at 12,630 ft (3,850 m). In 1939 the DB-3M (modernized) replaced the DB-3B in production. Through major structural redesign, this version needed about half the man-hours to produce. The DB-3T was a torpedo-bomber, carrying a 17.72 in (450 mm) 2,072 lb (940 kg) 45-36-AN (low level) or 45-36-AV (high-level) torpedo beneath the fuselage, with a small compressed-air tank behind for running up the torpedo before release.
One-offs were the DB-PT (Popla-vokovyi torpedonosets - Torpedo floatplane) twin-float version of the DB-3T. However, the floats and their struts were cumbersome and reduced speed by some 43.5 mph (70 kmh), and in the event the Navy used the landplane version. Another one was the DB-3GK, an unarmed high-altitude reconnaissance aircraft with a redesigned pressurized forward cabin and extended wingtips. In 1938 the sole CCB-54 tested revised armament with powered turrets, better ventral positions and improved machinegun sights. A single DB-3UPS had redesigned wings with slotted flaps/ailerons with boundary-layer suction from an 116 hp ZIS-101A engine/blower in the bomb-bay. The DB-3K was fitted with a releasable streamlined module (designated DK-12) for twelve para-troopers, reportedly both the TsKB-30 and a DB-3A were tested in this configuration. Excluding the DB-3F, a total of 1,528 DB-3s were built.
The DB-3 fuselage was a narrow oval-section semi-monocoque; late production Il-4s had external stiffeners on either side of the upper rear fuselage. The wings were built in three sections: center-section and two outer panels, and were attached at a slight dihedral angle. There was little leading-edge taper but trailing-edge taper was more pronounced. The ailerons were slotted and provided with servo and adjustable tabs. Divided split trailing-edge flaps were fitted. The DB-3 had large wing-root fillets. The tailplane was semi-elliptical in planform and the large fin had a mass-balanced rudder. Rudder and elevator trim tabs were fitted. The undercarriage main units, consisting of braced twin oleo legs retracted rearwards into the engine nacelles; the tail wheel was non-retractable.
The fuselage frame, wing ribs and stringers were made from welded rolled U-section steel strip. Frame type wing-spars were welded from chrome molybdenum tube. For a period during WW II, wooden spars were substituted when metals became scarce. Later, they reverted to metal construction. A duralumin stressed-skin covering was attached with snap headed (or round headed) rivets. The control surfaces were fabric covered. The DB-3's fuel supply was contained in ten tanks in the wings and totaled 951 gal (3,600 l); in addition, two supplementary tanks each of 106 gal (400 l) could be carried beneath the fuselage.
The DB-3 normally carried a crew of three, consisting of pilot, navigator/bomb-aimer/nose gunner and rear gunner. Armament consisted of a 0.30 in (7.62 mm) ShKAS machinegun in the nose turret (with 1,100 rounds), a single ShKAS in the dorsal turret (1,100 rounds), and a third which fired downward and rearward through a hatch in the fuselage floor (650 rounds). The normal bomb load was ten 220 lb (100 kg) bombs carried horizontally in two vertical banks in the center fuselage bomb bay; larger bombs had to be carried externally. For short-range operations, 2,204 lb (1,000 kg) could be carried internally together with up to 3,300 lb (1,500 kg), externally."