Though work on supporting items began immediately, actual design of the Tupolev ANT-25 did not start until April 1932. Tupolev appointed PO Sukhoi to lead the project. Structural design of the remarkable wing was entrusted to Petlyakov and Belyaev, with control surfaces assigned to N S Nekrasov. While the engine and its reduction gear were the responsibility of Mikulin, its installation and the fuel, oil and cooling systems were assigned to Ye I Pogosskii and K V Minkner.
The basis of the whole design was the remarkable cantilever monoplane wing with an aspect ratio of 13.1, and span 2.9 times the overall length. As before, it had a modified CAHI-6 aerofoil section, the thickness/chord ratio being 20 per cent at the root, 19.2 per cent at semi-span and 18.5 per cent just inboard of the pointed tip. Structurally it comprised a 3.75m (12ft 4in) horizontal centre section and two 15.125m (49ft 7in) tapered outer panels with 3 deg dihedral. As the chord was less than on most AGOS wings there were just two principal spars, at 18 and 44 per cent chord, plus a lighter rear spar. Most of the structure was the usual built-up diagonal and vertical lattice of D6 Dural, but the spar booms were of tube made from KhMA steel heat-treated to a u.t.s. of 140kg/sq mm. There were eighteen A-type ribs on each side projecting above the profile, with lighter intermediate ribs. Surprisingly, in view of the overriding need to reduce drag, corrugated skin was again used, the material having 40mm by 8mm corrugations riveted in strips between each pair of A-ribs.
The very long ailerons were made in four sections, each with a slotted nose and inset hinges giving 100 per cent mass balance. In the second aircraft (after modification) and third Tupolev ANT-25 the inboard two sections had chord increased behind the wing trailing- edge line, the outer of this pair also having a cable-operated servo tab.
The Tupolev ANT-25 engine chosen was the M-34, with compression ratio 6, rated at 750hp. There was direct-drive to a two-blade wooden propeller of 4.5m (14ft 9in) diameter. To minimise drag the water radiator could be fully retracted, or cranked down by the pilot along vertical slides immediately in front of the firewall to project only as much into the slipstream as necessary. Next to it was the big 350 litre (77gal) oil tank. In the wings were multiple riveted-AMTs fuel tanks between the spars housing a total of 7,640 litres (1,681 gal). Fear of wing flutter had led to prolonged research and testing, and the distributed fuel mass was found to play a major role in moving the flutter boundary away from normal flight conditions. On the other hand, at a time when fatigue was totally ignored in Britain and some other countries, careful fatigue tests were made, and the design factor reduced from 4.8 when new to 3 after 1,000 hours.
Accommodation in Tupolev ANT-25 was provided for a pilot, radio operator/navigator and flight engineer in tandem cockpits sealed against the weather. All three cockpits had hinged glazed roofs and side windows.
Test flying the RDD, also called Tupolev ANT-25D, began on 15 June1 936. Range, cruising at 5,500m (18,050ft) with 3,500kg (7,7161b) of fuel, was found to be 10,800km (6,711 miles). A retractable undercarriage would have extended this to about 12,000km (7,457 miles). The diesel engine was usually difficult to start. In 1940 Charomskii’s later M-40 diesel powered the BOK-11, one of several BOK aircraft based on the ANT-25 airframe. An AN-1 was installed in at least one ANT-36.