Running-up the Hispano-Suiza engine of the Blackburn T.7B in the presence of Mitsubishi representatives, Brough, December 1929.
The Blackburn-built 3MR4 prototype in Japan with Japanese Navy markings and two 113.3 kg (250 lb) bombs between the undercarriage legs.
In a competition that began in February 1928, the Navy asked for proposals from Aichi, Kawanishi, Mitsubishi and Nakajima for a design for a new carrier attack bomber to replace the Mitsubishi Type 13 Carrier Attack Aircraft, (BIM3). The Navy stipulated a crew of three, the 600 hp Hispano-Suiza, 450-600 hp BMW or the 600-650 hp Lorraine engine and that the structure was to be of mixed wood and metal construction. The span was to be less than 15 m (49.2 ft), length less than 10 m (33 ft), and height less than 3.8 m (12.4 ft).
Performance asked for was reasonable for that time, with a maximum speed of more than 110 kt (126.5 mph) at sea level, climb to 3,000 m (9,843 ft) in 15 minutes, and a ceiling of over 6,000 m (19,685 ft). Endurance was to be more than three hours with a load of bombs, or more than eight hours without bombs. The deck landing speed was to be less than 45 kt (52 mph), and take-off distance less than 45 m (148 ft) with a surface wind of 20 kt (23 mph).
Mitsubishi sub-contracted each different design study to three different teams. The first design termed 3MR3 was engineered by Herbert Smith, who had returned to England in June 1924. Mitsubishi decided to use an even newer engine, the 650 hp Armstrong Siddeley leopard. The company sub-contracted a second design study termed 3MR4 to the Blackburn Company in Britain, powered by the 600 hp Hispano-suiza. The third design study, the 3MR5, was sub-contracted to Handley Page Company, also in Britain. This was to have a 600 hp Hispano-Suiza engine like that of the 3MR4. Of the three design proposals, Mitsubishi selected the Blackburn-designed 3MR4 as the best and submitted it for the competition. The Navy declared this aircraft to be the design winner in December 1928, and Mitsubishi therefore directed Blackburn to manufacture the first prototype.
Before the order was placed, Mitsubishi sent engineer Hajime Matsuhara to England to gain technical knowledge on aircraft engineering planning and design, three additional engineers, Arkawa, Yui and Fukui, were also sent to Blackburn to learn the fundamentals that were to be incorporated into the 3MR4.
The Blackburn T.7B (or 3MR4), was a two-bay staggered biplane with Frise ailerons on all four wings and equipped with Handley Page leading edge slots, but the lower mainplane was some 0.33 m (13 in) greater in span than the upper and the outer interplane struts were not parallel to the inner. The fabric-covered all-metal wing structure followed standard Blackburn practice, using high-grade steel box spars built up from specially rolled and drawn sections in conjunction with duralumin ribs, the outer panels being hinged to the centre section rear spars. They folded with the aid of a jury strut, but the lower centre section, being of greater span than the upper, imparted some 45 degrees of tilt to them when folded.
The fuselage was of the usual Blackburn three-piece weldless steel-tube construction faired by aluminium panels to a point aft of the cockpit for ease of servicing and re-arming. The fabric-covered rear section was filled with flotation bags except for the stern bay, which housed the ballast weights used for C G adjustment when changing from three to two-crew operation.
Although larger, the divided oleo-sprung undercarriage and rectangular tail unit were similar in design and construction to those of the Beagle, and the tailplane was adjustable in the air by means of a handwheel on the starboard side of the cockpit. The tail skid was oleo-pneumatically sprung from the sternpost, and attachment points for a float undercarriage were an integral part of the fuselage structure although floats were in fact, never fitted.
The closely-cowled 600 hp Hispano-Suiza 51-12Lb twelve-cylinder vee water-cooled engine drove a two-blade wooden propeller and was mounted on duralumin bearers supported on a steel-tube structure, but for simplicity the old Swift/Dart/Velos cooling system was revived using a radiator compartment under the engine but with vertical instead of horizontal shutters. Total fuel capacity was 918.3 litres (202 Imp gal) housed in two 168.2 litres (37 Imp gal) gravity tanks in the top centre section, a 200 litres (44 Imp gal) tank between the pilot and the fireproof bulkhead, and a fourth of (84 Imp gal) capacity below the pilot's floor. For long-range operation a 386.4 litres (85 Imp gal) streamlined overload tank with nose-mounted, wind-driven fuel pump, could be carried in the torpedo crutches. A 41 litres (9 Imp gal) oil tank was fitted under the decking ahead of the pilot and the coolant header tank was in the centre section as on the Ripon.
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