Tupolev ANT-36 / DB-1
The ANT-36 was a redesigned ANT-25, and the twenty aircraft built for the Soviet Air Force under the military designation DB-1 were actually ANT-36s. This has caused some confusion as the sometimes reported production figures of the ANT-25 are quoted as two (the correct figure) and twenty-two, the total which includes both the -25 and the -36.
The major differences between the two Sukhoi-led programmes was armaments: on two aircraft, the Mikulin M-34R of the ANT-25 was changed for a Junkers Jumo 4 diesel and, later, the Soviet-built AN-1 diesel. But these engines left the DB-1 underpowered and slow, and its limited armaments, low service ceiling and large wing meant it was very vulnerable to fighters, so the planned production of fifty was cut to twenty which were never to see service in the role for which they were intended.
By spring 1936, these twenty had been delivered to the Air Force base at Ismailova, near Moscow. Two of these were fitted with diesel engines, and flight tests indicated that this version could achieve a range of 25,000km. It was planned to circumnavigate the world on the 57°N line of latitude, but the build-up to the Second World War caused the plans to be put aside. The remaining thirty aircraft were not built.
The aircraft was not a military success. Labelled the DB-1, the Air Force soon realised that the almost five years it took to develop had left the aircraft too slow, and that against contemporary fighters it was outclassed and virtually defenceless. But the twenty military aircraft went on to further flight trials; two were fitted with Junkers Jumo diesel engines, as has already been mentioned. Most of the others were used by the TsAGFs BOK, a department headed by Vladimir Chizhevski which was developing pressurised cabins for high-altitude test flights. In 1936, Chizhevski had worked in Kharkov developing pressurised cabins for high-altitude balloons. He lightened the airframe and shortened the span for his first version, the BOK-1. The engine chosen was the M-34RN with a turbocharger fitted to permit a ceiling of 10,000m to be attained. In early trials Piotr Stefanovski reached 10,700m; later, after further lightening of the aircraft, he brought it up to 14,100m. In June 1937, the engine was replaced by the new M-34RNV, fitted with two turbochargers. With some lightening, the aircraft reached heights of over 12,000m.
In 1938, Chizhevski modified another ANT-36 to BOK-7 standard. He substantially modified the cabin, so now the two crew sat with their heads in small doubleglazed domes which rose above the line of the fuselage. It was powered by a M-34FRN and had two superchargers. With this, the Soviet Air Force set its plans for a non-stop round the world flight approximately on the 53° North line of latitude. The programme was headed by Aleksander Filin, one of Chkalov's crew, and targeted for 1939 or 1940. But Filin was arrested in Stalin's purge, and executed in 1940. This and the war ended the project.