Staff Writer (Updated: 5/22/2016):
Design-wise, the aircraft was of a sleek design. All four engines (of types that varied throughout the course of development and the war itself) were mounted high atop the wing assembly which sat on the fuselage via struts. Pontoons adorned either wing for water-based operations and the boat-like hull could clearly be distinguished. The fuselage was long and slender ending in a twin vertical tail design. Accommodations amounted to 9 personnel and positions included defensive armament consisting of 4 x 7.7mm machines found in the open bow, dorsal and two beam blister positions. A 20mm cannon turret was seated at the extreme rear of the aircraft. Offensive armament could amount to over 4,400 pounds of ordnance or 2 x 1,764 pound torpedoes for anti-shipping duty.
The H6K1 was the initial service model to enter production but this service was limited in 1938. The first major production model series became the H6K2 and that was followed by the solid addition of the H6K4 which sported 4 x Mitsubishi Kinsei 43 radial engines (later production models featured the Kinsei 46) and improved defensive armament. Dedicated transport derivatives were designated by an "L" and were spawned from the H6K2 and the H6K4 models. As these models went to war, it was found that they were highly susceptible to enemy fire and as such the H6K5 series was developed.
The H6K5 featured self-sealing fuel tanks and better armor protection for the crew and systems alike. Additionally, the open bow gun position was altogether removed in favor of a covered dorsal machine gun turret above the cockpit flight deck. Despite these improvements, the H6K was never really up to the demanding and changing tasks of the Pacific War and thusly production of this model was limited.
As the production line evolved and progressed during war time, the H6K "Mavis" also took on the codename of "Tillie" for the Allies to represent the dedicated transport derivatives of the H6K series. Some 215 total H6K's were produced by and for Kawanishi. Japan ceased operation of the type in 1945 whilst Indonesia gave up operations in 1948. The H6K was replaced in production by the more capable Kawanishi brand H8K series of flying boat, detailed elsewhere on this site.