The matter of a replacement for the H6K Mavis was a challenge - the Navy in 1938 producing a specification calling for 30 per cent higher speed and 50 per cent greater range.
In the H8K Kawanishi produced a design which represented the biggest single leap in the design of such aircraft in all of aviation history. It was beyond dispute the best, most advanced, flying boat in the world until years after World War Two. A notable feature of the H8K was its exceptionally powerful defensive armament.
However, the H8K's early trials were disastrous - primarily because the aircraft's great weight and narrow-beamed hull produced uncontrollable porpoising. The cure was found in adding a second step in the planing bottom, adjusting the powerful double-slotted Fowler flaps, and adding a horizon mark on the large pitot post above the bows. The aircraft's first flight took place in late 1940, and the H8K1 went into production in August 1941.
The allies gave the H8K the reporting-name 'Emily'. In all the Kawanishi plant at Kohnan built 17 H8K1, 114 H8K2 and 36 H8K2-L(a transport version). These powerful flying boats ranged the Pacific on many audacious 24-hour missions, and proved formidable. Later versions were equipped with radar, and some aircraft were provided with retractable stabilising floats.
3-View of Kawanishi H8K2
Origin: Kawanashi Kokuki KK
Type: Maritime patrol, reconnaisance and attack flying boat
Dimensions: Span 124' 8" (38 metres) Length 92' 4" (28.1 metres) Height 30 feet (9.15 metres)
Weights: Empty (H8K1) 34,000 lb (H8K2) 40,500 lb Loaded (H8K1) 68,343 lb (H8K2) 71,650 lb
Engines: Four Mitsubishi Kasei 14-cylinder two-row radials
Maximum speed 280 mph
Initial climb 1,575 feet (480 metres) per minute
Maximum ceiling 28,800 feet (8,770 metres)
Normal range 3,000 miles (4,800 kilometres)
Overload range for reconnaisance 4,474 miles (7,200 kilometres)
Five 20mm cannon in power-operated forward, ventral and tail positions
Three 7.7 mm machine-guns manually aimed from beam and ventral rear windows
Up to 4,400 lb of bombs or torpedoes carried under wings
Main source for the above text was Bill Gunston's book
"Combat Aircraft of World War Two" (published by Spring/Salamander)
The colour profile and the 3-view of the H8K2 are reproduced from the same publication