Kawanishi was a leader in flying boat development and had gained most of its experience in designing large sea going aircraft. Two of their most successful designs were the Kawanishi H6K (codenamed Mavis by the Allies) and the H8K (Emily), with the latter arguably the best flying boat of World War 2. The company was also not lacking in cargo flying boat design having modified the H6K to serve as a transport as the H6K2-L and H6K4-L. Even the H8Kwas adapted as a transport, the H8K2-L Seikū (or Clear Sky). Kawanishi had also been working on the design of the K-60, a long-range transport flying boat. With these credentials, Kawanishi was able to capitalise on their knowledge to begin the design of the H11K Soku (Blue Sky) for the IJN.
Kawanishi was instructed by the UN to use as much wood as possible in the construction of the Soku since a flying boat of such size would have consumed a large amount of precious alloys needed for other aircraft such as fighters. Within Kawanishi, the Soku was called the KX-8 and the initial design draft was processed rapidly. The aircraft drew heavily from the H8K being a high-wing, cantilever monoplane but overall, the Soku was much larger. The keel of the Soku was nearly identical to the H8K. To power the flying boat four Mitsubishi MK4Q Kasei 22 (Ha-32-22) radials, each developing 1,850hp, were selected with two per wing. As ordered by the UN, both the fuselage/hull and the wings were to be built of wood and under each wing would be a non-retractable float. The Soku had two decks. The lower deck could accommodate up to eighty fully equipped soldiers including a number of vehicles or a comparable amount of cargo. A smaller, upper deck housed quarters for the crew of five. The main departure from the H8K transports was that the Soku utilised a split nose that was hinged to allow the two nose sections to be opened outward to each side of the fuselage, providing ready access to the lower deck. This facilitated easier loading and unloading increasing the speed and ease of these procedures. As a measure of protection the Soku was to be fitted with three 13mm Type 2 machine guns.
Kawanishi presented the KX-8 to the IJN and the design was accepted. Authorisation was given to construct a full scale wooden mock-up of the Soku now designated the H11K1 for inspection before Kawanishi could proceed with the actual prototype. Construction of the mock-up commenced at the port of Komatsujima in the city of Komatsushima on the island of Shikoku (the smallest of the four main islands making up Japan). This area was selected by Kawanishi because it had access to the Seto Inland Sea which, once the prototype was built, would be needed to undertake sea and flight trials. Unfortunately for the Soku, the deteriorating war picture saw delay after delay affect the construction of the mock-up. To add to the problem, Kawanishi was instructed by the IJN in 1945 to reduce production of the H8K and instead, concentrate on building the Kawanishi NI K2-J Shiden-Kai fighter. Together, these factors would see the mock-up approaching its completion in April 1945, well over a year after the design had been initiated.
On 1 April 1945, bombing raids conducted on targets along the Seto Inland Sea saw the nearly completed Soku mock-up destroyed. With this loss, all further work on the Soku design was shelved.
Type Transport Flying Boat
Powerplant: Four Mitsubishi MK4Q Kasei 22 (Ha·32-22) 14-cylinder, air-cooled radial engines developing 1,850hp for take-off, 1,680hp at 6,886ft and 1,550hp al 5,500m/18,044ft: each engine drove a 4.3m (14.lft) diameter, four-bladed, alternating stroke propeller
Span: 47.97m 157.4ft
Length: 37.70m I23.7ft
Height: 12.55m 41.2ft
Wing area: 289.95m2 3,121ft2
Wing loading: 156.72kg/m2 32.1 lb/ft2
Power loading: 6. I2kg/hp 13.51blhp
Empty: 26,405kg 58,213lb
Loaded: 45,550kg 100,420 Ib
Useful load 19,095kg 42,097lb
Max speed: 470km/h 292mph at 5,000m at 16,404ft
Cruise speed: 369km/h 229mph
Landing speed: 144km/h 89mph
Range: 3,890km 2,417 miles
Climb: 11 min 30 sec to 3,000m (9,842ft)
Armament: Three 13mm Type 2machine guns with 200 rounds of ammunition per gun