Meet the world's first cantilever wing four-engine heavy bomber - the Tupolev ANT-6 aka TB-3.
In 1925, the Soviet Air Force approached TsAGI with a requirement for a heavy bomber with total engine output of 2,000 PS (1,970 hp) and either wheeled or float landing gear. Tupolev OKB started design work in 1926 with the government operational requirements finalized in 1929.
Tupolev TB-1 (above) was taken as the basis for the design and the aircraft was initially powered by Curtiss V-1570 "Conqueror" engines generating 600 PS (590 hp) each, with the intent of switching to Mikulin M-17s (modified BMW VIs) in production.
The prototype was flown first on 22 December 1930 and production began at the end of 1931, continuing through many modifications until early 1937 when a total of 818 had been built. For many years the TB-3 was the backbone of the VVS (Soviet air forces) heavy bomber units.
In 1933, a single TB-3-4M-17F was streamlined with removal of turrets and bomb shackles, covering of all openings, and fitting of wheel spats. This resulted in only a 4.5% increase in top speed and a similar increase in the range. Tupolev concluded that streamlining was minimally beneficial for large and slow aircraft. To study the effect of corrugated skin, in January–February 1935 a single TB-3-4AM-34R had the corrugations incrementally covered with fabric. This resulted in a 5.5% gain in top speed and a 27.5% increase in the ceiling. The same aircraft demonstrated a significant increase in climb rate when fitted with experimental four-blade propellers.
A number retained the bureau designation ANT-6 and were used for transport, particularly in the Arctic. In 1938-39 TB-3s were used operationally against the Japanese, but by the time Germany invaded the Soviet Union in 1941 most had been converted as paratroop or freight transports under the designation G-2.
Although it was officially withdrawn from service in 1939, at the start of the German invasion (June 22, 1941), the Soviet Air Force had 516 operational TB-3s, with an additional 25 operated by the Soviet Navy. The TB-3 served extensively as a cargo and paratroop transport, carrying up to 35 soldiers in the latter role. In the first five months of the war, the aircraft transported 2,797 tons of cargo and 2,300 personnel.
On 1 August 1941, a pair of TB-3s in Zveno-SPB configuration, each with two Polikarpov I-16 fighters carrying a pair of 250 kilograms (550 lb) bombs, destroyed an oil depot with no losses. On 11 August and 13 August 1941, Zveno-SPB successfully damaged the King Carol I Bridge over Danube in Romania. Zveno operations ended in the fall of 1942 due to high vulnerability of the motherships. Other uses then consisted of night bombing and transport work of all kinds, including the carriage of vehicles or tanks between landing gear legs, and glider towing. Use in parasite fighter experiments led in 1941 to Black Sea Fleet TB-3s being used to launch two Polikarpov SPB dive-bomber versions of the I-16 fighter, for raids on pinpoint targets in the Ukraine and Romania.