ANT-37 (DB-2) record-breaking aircraft and long-range bomber

In 1934 the VVS began to change its views on the long-range bomber. Technical requirements now specified that the new bomber should carry a 1,000-kg bomb load over no less than 3,000 km and have a speed at rated altitude of no less than 350 km/h.
In the autumn of 1934, on the basis of these new requirements, Tupolev was instructed to produce a long-range bomber with the OKB designation ANT-37 and the service designation DB-2. The prototype was to be ready for testing by the summer of 1935. As in the case of the DB-1, the new project was to be handled by the team headed by Sukhoi.
The project took the aerodynamic layout of the ANT-25 as its starting point, with a high wing aspect ratio (11.3) and a relatively low wing loading for its area. During the prototype's construction, it was decided to use some airframe parts and assemblies from the ANT-36. However, unlike the ANT-25, the ANT-37 was designed as a twin-engined monoplane powered by two 800-hp Gnome-Rhone Mistral Major K-14 radials (built in the USSR under licence as the M-85). The normal internal bomb load was to be 1,050 kg, increased to 2,050 with the fitting of external bomb racks. The defensive armament was to consist of three 7.62-mm ShKAS rapid-firing machine-guns. The design range was around 5,000 km at a speed of 250 km/h.
The mock-up was approved in January 1935, construction of the first prototype being completed on 15th June. The following day the machine was taken to the airfield for manufacturer's flight tests performed by pilot K. K. Popov and chief engineer M. M. Yegorov. When the tests ended on 15th July the prototype was handed over for state trials. Then, on 20th July, the ANT-37 crashed; Popov and Yegorov escaped, but electrics engineer Titov lost his life. The cause of the accident was vibration in the tail assembly causing the rear fuselage as far as the co-pilot's position to break away. The crash triggered further research work on flutter and buffeting problems.
In July-August 1935 construction of the second prototype began. This differed from the first in having wing fillets and a stronger fuselage. It was finished on 18th February 1936 and made its first flight exactly a week later. After manufacturer's tests had been completed, the dooblyor piloted by M. Yu. Alekseyev made a non-stop flight from Moscow to Omsk and back with a bomb load of 1,000 kg, covering a distance of 4,955 km in 23 hours and 20 minutes at an average speed of 213 km/h. On 19th February 1937 the machine was submitted for state trials by Nii VVS, during which tail vibration was detected at relatively modest speeds of 140-150 km/h and further flights were suspended. All further work on the aircraft in its bomber form was terminated.
In parallel with the construction of the two prototypes, preparations for the series production of 30 machines had been made at Factory No. 18. Later, given the possible extension of the production series, a civil version of the aircraft was proposed. With the active participation of the Tupolev OKB, construction of two pilot production examples was begun, but in early September 1936 work on these stopped and they were placed at the disposal of the OKB. A third example of the DB-2, 52% complete at Factory No.18, was modified at Factory No.156 in May 1937 into a long-range record-breaking machine capable of covering 8,000 km. It was planned to use a fourth example with wing tanks for flights over a range of 12,000 km but the machine was not completed.
When the first production example was rebuilt as the record-breaking ANT-37bis (DB-2B) or Rodina (Motherland), it was fitted with more powerful 860/950-hp M-86 engines, three-blade variable-pitch propellers (as fitted to the second prototype in the course of its state trials), a reconfigured nose section and additional fuel tanks. All the armament was removed.
On 24th-25th September 1938 the Rodina flown by captain Valentina S. Grizodoobova with co-pilot Polina D. Osipenko and navigator Marina M. Raskova set up a new women's world non-stop long-distance record. Taking off from Moscow, the ANT-37bis covered 5,947 km (5,908 km in a straight line) in 26 hours and 29 minutes at an average speed of 224 km/h, eventually making a wheels-up landing not far from Komsomol'sk-na-Amure. After repair, the aircraft was subsequently operated by Aeroflot and then, until 1943, by the People's Commissariat of Aircraft Industry (NKAP - Narodnyy komissariaht aviatsionnoy promyshlennosti).