From Robert C. Mikesh and Shorzoe Abe's book Japanese Aircraft 1910-1941 (Putnam Aeronautical Books, 1990), pages 100-101:
Navy Type 95 Land-based Attack Aircraft (G2H1; Navy Experimental 7-Shi Attack Aircraft)
"At the Washington (Disarmament) Treaty of 1922 limited the tonnage for capital ships for the US Navy, the Royal Navy, and the Japanese Navy, so did the London (Disarmament) Treaty of 1930 limit the number of smaller ships including aircraft carriers and cruisers. Japanese Navy planners recognized the capability of Navy land-based bombers that could be used to supplement and reinforce fleet activities and thus were responsible for the development of the Hiro Navy Type 95 Land-based Attack Aircraft.
"To meet this new requirement for air power starting in 1932, Rear Admiral Isoroku Yamamoto, Chief of Engineering Department, Naval Air Headquarters, called for a land-based long-range attack bomber that could fly more than 2,000nm and carry two tons of bombs. The Hiro Arsenal was selected for the project, for at that time it was the most experienced in the design of all-metal large aircraft. Chief designer was Lieut-Cdr (Ordnance) Jun Okamura who had served in this capacity for the preceding Type 91 flying-boat project. This land-based bomber became the primary concern for the Hiro Arsenal, diverting attention from the development of the flying boats previously described.
"At the start of the project, the prototype's designation was the Hirosho 7-Shi Speical Attack Aircraft, with the short designation G2H1. Structurally, it was a combination of a large wing of traditional Wagner diagonal tension-field structure and a slender fuselage of monocoque construction. The twin fins and rudders were similar to the final design of the Type 90-1 Flying Boats, and the ailerons were of the Junkers double-wing variety. One of the innovative features of the armament installation was a cylindrical belly turret which retracted into the fuselage. This feature was carried over into versions of the Mitsubishi Navy Type 96 Land-based Attack Aircraft, that were code-named Nell during the Pacific War.
"To power the new bomber, two 900-1,180hp Type 94 water-cooled engines were selected, the most powerful aircraft engines available at that time. They were being developed by the Hiro Arsenal as a scaled-up version of the 600hp Type 90 engine. It was felt that with these new engines, the aeroplane would be equivalent to a three- or four-engined aircraft of the time. Although the airframe dimension, wing area, and empty weight were almost identical to the Type 90-1 Flying Boat, aircraft range and payload were increased by nearly 50 percent. This was the largest land-based aeroplane in the Navy at that time, second only to the Army's Type 92 Heavy Bomber (Ki.20) of the Junkers-G 38 design, yet it was the first of such a large size to be designed from the beginning as a land-based attack bomber. With two engines, its wing span was 103ft 11-1/4in, marginally bigger than the four-engined Boeing B-17 Flying Fortress with 103ft 9in wingspan.
"The first prototype was completed on 19 April 1933 at the Hiro Arsenal and moved by ship to Yokosuka. There it made its first flight in mid-May 1933 in the presence of Rear Admiral Yamamoto who had originated this bomber concept for the Navy. Making the first flight were Lieut-Cdr Shinozuke Muneyuki and Lieut-Cdr Toshihiko Odahara, both of the Flight Experimental Group of the Yokosuka Kokutai. After taking off, Muneyuki made one pass over the field for the spectators and proceeded to Kasumigaura AIr Base where testing was to take place.
"As flight evaluations continued, it was found that the aeroplane possessed outstanding performance as the Navy's largest land-based aeroplane at that time. But shortcomings became evident, including tail vibrations caused by the light structure of the fuselage, aileron flutter, and unreliable engines. One aircraft was lost during test flying because of aileron and tail flutter, causing it to ditch in Tokyo Bay. Corrections were made to the design enough to justify production.
"In June 1936, the aeroplane was officially accepted by the Navy as the Type 95 Land-based Attack Aircraft, at the same time as the Navy accepted the Type 96 Land-based Attack Aircraft (G3M1) [later "Nell"]. To avoid identity confustion between the two, the G3M1 was referred to as the Type 96 Chu-Ko (Medium Attack) or simply 'Chu-ko', while the G2H was called the Type 95 Dai-ko (Large Attack) or 'Dai-ko'.
"After six of the G2H bombers had been produced at Hiro Arsenal, production was transferred to Mitsubishi. Before long, however, the Navy asked that production be concentrated on the smaller G3M, curtailing the G2H because of maintenance difficulties with the Type 94 engines and the aeroplane's low-speed flying characteristics. Consequently, production ended with only two having been manufactured by Mitsubishi.
"With the activation of the Kisarazu Kokutai on 1 April 1936, all remaining G2H1s (a total of eight were built) were assigned to this unit but were regarded as second-line aircraft because of the better performance of the G3Ms.
"Heavy losses were experienced by G3Ms over Nanjing in August 1937, resulting in the deployment of the G2Hs to an airfield on Saishuto Island (now Cheju Do, off the southern coast of South Korea), and while en route, and for unexplained reasons, one G2H dropped out of formation and crashed near the coast of Sagami Bay southwest of Tokyo. Once in place, and established as the 1st Combined Kokutai with other forces from Kanoya, they made their first mission into China in support of ground forces in the Shanghai area on 30 September 1937 under the command of Lt Motokazu Mihara. They made further attacks against nine major combat areas and received considerable damage from AA fire, but no aeroplanes were lost.
"Disaster did catch up with these G2Hs on 24 October 1937, when one aircraft caught fire while its engines were being started and soon exploded. The fire spread to the other G2Hs, each loaded with three 250kg, five 60kg, and five 50kg bombs, exploding successively until four aircraft were destroyed and the fifth badly damaged.
Twin-engined land-based mid-wing monoplane bomber. All-metal stressed-skin construction. Crew of seven.
Two 900-1,180hp Hiro Type 94-1 eighteen-cylinder W-type water-cooled engines, driving four-bladed wooden propellers.
One nose-mounted flexible 7.7mm machine gun, twin dorsal 7.7mm machine guns retractable turret-mounted, one retractable turret-mounted 7.7mm machine gun. Bombload: six 250kg (551lb) bombs or four 400kg (881lb) bombs.
Span 31.68m (103ft 11-1/4in); length 20.15m (66ft 1-1/4in); height 6.28m (20ft 7-1/4in); wing area 140sq m (1,506,996sq ft).
Empty weight 7,567kg (16,682b); loaded weight 11,000kg (24,250lb); wing loading 6.11kg/hp (13.4lb/hp).
Maximum speed 1332kt (152mph) at 1,000m (3,280ft); cruising speed 90kt (104mph); climb to 3,000m (9,843ft) in 9min 30sec; service ceiling 5.130m (16,830ft); range 1,080 to 1,557nm (1,245 to 1,800nm).
Hirosho built six from 1933 and Mitsubishi built two from 1936.
"Thus, Hirosho closed its impressive history of all-metal aircraft development. Its refinement of the technology, with improvements over the imported Rohrbach designs, was passed to Mitsubishi which led to its success with the 9-Shi Single-seat Fighter (A5M "Claude") and the 8-Shi Special reconaissance aircraft (that developed into the G3M Nell), and brought Japanese aeronautical engineering capability to Western levels. Once this success was recognized, the new technology spread to all other Japanese aircraft manufacturers, for both the Army and the Navy.
"From 1935, the Navy continued to expand its aircraft repair and supply facilities. At Hirosho, new ground was added by cutting down mountains and reclaiming coastal land, and ultimately branch arsenals of Hirosho were established at Ohita and Maizuru. From 1 October 1941, all aircraft departments of Naval arsenals became Naval Air Arsenals. These were different to the former Air Arsenals which had already been renamed Naval Air Technical Arsenals. WIth this reorganization, Hirosho became the 11th Naval Air Arsenal.
Hirosho began augmenting production of aircraft types developed by other manufacturers in an effort to supplement the buildup of the war effort. Among these principal types were the Nakajima Navy Type 97 Carrier Attack Aircraft (Kate) and Aichi Navy Type 0 Three-seat Reconaissance Seaplane (Jake), followed by the Aichi Navy Carrier Bomber (Judy) and various Navy engines from commercial manufacturers. No other aircraft after the Type 95 Attack Aircraft (G2H) of hte mid-1930s was identified as a Hirosho-designed aircraft, since the function of new aircraft development was transferred to Kugisho.*
* The last aircraft with Hiro code H was the H10H1, 14-Shi Medium Flying boat, but this was not completed because of higher wartime priorities.