This large flying boat was included into plan of the TsKB naval section (Morskoj Otdel)
under No11. Designers decided to use wing and some other parts from the TB-5 heavy bomber. In January
1931 the first in the country hydrochannel trials were performed on the MDR-3 scale maquette. Shape of the
hull was found successful, what was proved lated during aircraft trials.
Assembly started in Spring 1931 and aircraft was ready in December same year. It was large shoulder-wing
monoplane (with struts) and balance floats (3m3 each) placed relatively close to the hull. Four liquid-cooling engines were arranged in two
tandems and installed on struts above the wing. Each pair had common water radiator under the front engine. Installation was very convenient for service
purposes. Upper surface of central wing section was made of corrugated aluminum to allow to access engines. Hull was similar to that of
ROM-2, but with additional step and external stringers.
Equipment of pilot's and navigator's cockpit, gun installations were same as on TB-5, with addition of
specific naval equipment (anchor, rafts). Two 250kg bombs could be installed on fuselage hardpoints (between the wing struts).
In January 1932 aircraft was transported to Sevastopol Naval Base. Test flights started
on January 13 by B.L.Bukhgolts and finished on March 25 same year. Aircraft was rejected...
MDR-3 had good seaworthiness, short takeoff run and decent for its class speed, but climb rate and ceiling were unacceptably poor. Partially it could
be blamed on tandem installation of engines, but main trouble was poor aircraft airdynamic. Engine tandems were too close to the wing (specially the
radiators). Numerous rivets with semi-spherical heads, other 'spiky' parts in the air flow, missing fairings on struts, external elements of the hull frame
all together contributed to poor performance.
MDR-3 was transferred to A.N.Tupolev, who transformed it into MDR-4.