Nakajima Ki-49-II Donryu Army Bomber, type 100 model 2 (code name "Helen")
In the spring of 1942, it was decided that the Ki-49 should be fitted with new engines, the Nakajima Ha-109 radial piston engines, rated at 1450 hp. This required only a slight modification of the engine nacelles, as the new engine was a similar size to that of its predecessor. However, the oil cooler could not fit inside the nacelle along with the engine and was put outside, instead. Other changes were made to improve combat conditions: the armor protecting the airplane's crew was upgraded and self-sealing fuel tanks were installed, as well as a new bomb sight.
The defensive armament remained similar to that of the Ki-49-I, but the Type 89 machine guns were replaced with Type 98 7.92mm machine guns (which were based on the Rheinmetall-Borsig MG.15). Normal bomb carrying capacity was increased to 1000 kilograms.
In August of 1942, the new version of the Donryu entered production, under the designation Ki-49-IIa (model 2Ko). However, the fighters' machine guns were soon proven to be ineffective against Allied fighters, so a new modification, the Ki-49-IIb (model 2Otsu) was designed with stronger defensive weaponry. In the bow, stern, and lower firing locations, the Type 98 7.92mm machine guns were replaced with Ho-103 12.7mm machine guns (the Type 1 Japanese version of the Browning M2.50). The two guns on the aircraft's sides were replaced with the original Type 89 7.7mm machine guns. The bomb load remained unchanged.
In later series, the Ki-49-IIb's common exhaust was replaced with several separate pipes.
The Ki-49-II bombers were unable to completely replace the older models in service. They were used mainly in New Guinea and China, and, in October of 1944, they took active participation in the Battle of Leyte.
The Ki-49 was used as both a day and a night bomber as well as for long-range reconnaissance, transport, and anti-submarine patrols. Towards the end of the war, they were used in kamikaze units. For kamikaze missions, the Ki-49 was stripped of all defensive armaments, the crew was reduced to two pilots, and the bomb load was increased to 1600 kilograms.
Production of the Ki-49 ceased in December of 1944. In all, 819 Ki-49s were constructed, including all variants.