The Nakajima Ki-49 was designed as a replacement for the
Mitsubishi Ki-21, which was just entering service in the spring of
1938. The Japanese Army Air Staff issued the specification at that
very time, calling for a bomber capable of operating without
fighter escort, relying instead on heavy defensive armament and
high speed to escape interceptors. A maximum speed of 311 miles per
hour was requested, an improvement of about 16 % over the Ki-21.
Also included in the requirements were a range of 1,864 miles, a
bomb load of 2,205 pounds, and a heavy (for the time) defensive
armament including one flexible 20mm cannon in the dorsal position
and several flexible 7.7mm machine guns, including a proper tail
turret (the first ever fitted to a JAAF bomber). Interestingly, the
specification called for crew armour and self-sealing fuel tanks-a
very welcome advance, considering the disdain the Japanese normally
showed for aircrew and fuel-tank protection.
When Nakajima lost the bomber competition in 1937 to Mitsubishi,
they had received a contract to build the Ki-21 in lieu of their
own Ki-19, and the Ki-49 design team, led by T. Koyama, used the
opportunity to study their competitor’s aircraft with a view toward
improving on it when they designed its intended replacement.
Especial attention was paid to handling, and a mid-mounted wing of
low aspect ratio was selected to ensure good manoeuvrability and
stable flight at low and medium altitudes.
The centre section of the wing, inboard of the engine nacelles,
was wider in chord than the outer panels to accommodate six
self-sealing fuel tanks, three on each side of the fuselage. This
also reduced drag and allowed the nacelles to be positioned well
ahead of the wing trailing edge. To improve take-off and initial
climb, Fowler flaps were adopted, and they extended from the
fuselage to the ailerons. In each of the outer wing panels, two
more self-sealing fuel tanks and a protected oil tank were fitted.
The defensive armament was a single 20mm cannon mounted flexibly in
the dorsal position and a flexible 7.7mm machine gun in each of the
nose, ventral, port and starboard beam, and tail positions. The
bomb bay was large, extending almost the entire length of the wing
centre section. All in all, the new bomber looked very impressive
when it was completed and flown for the first time in August
1939.
Powered by two 950-hp Nakajima Ha-5 radials, this first example
was used primarily for handling trials; service pilots were
particularly impressed with the prototype’s manoeuvrability, but
complained that the Ki-49 was rather underpowered when actually
carrying a bombload, and was harder to fly than the Ki-21. The
second and third prototypes, powered by two Nakajima Ha-41s of
1,250 horsepower, were completed and flown in the final quarter of
1939. These were followed by seven pre-production machines in 1940,
and throughout that year and into the beginning of 1941, all ten
Ki-49s were extensively tested. After minor modifications in
protection, armament, and seating arrangements, the type was
formally adopted by the Army in March 1941 as the Army Type 100
Heavy Bomber Donryu (Storm Dragon) Model 1 (Ki-49-I).
While the service tests were proceeding with the first ten
examples, shocking reports were coming in from the Chinese
battlefront. The Ki-21 was suffering heavy losses because the
Army’s then-standard fighter, the Nakajima Ki-27, lacked sufficient
range to accompany the bombers all the way to and from their
targets. This created a delay in delivery of the Ki-49 to
front-line units, for valuable time was wasted in trying to make a
heavy escort-fighter version of the Donryu, the Ki-58.
Three prototypes were built between December 1940 and March
1941. The Ki-58 resembled the Navy’s G6M in that it had the bomb
bay sealed and replaced by a ventral gondola; further armor
protection was fitted, and the defensive armament was increased to
no less than five flexible 20mm cannon and three 12.7mm machine
guns. Fortunately, the idea was abandoned when tests of the new
Ki-43 Hayabusa proved it to have the range needed to be a proper
escort fighter. But further effort was diverted into the building
of a “formation leader’s” aircraft, the Ki-80; two prototypes of
this variant were built in October 1941, but further work on the
Ki-80 was cancelled by the coming of the war. The two prototypes
eventually were used as test-beds for the 2,420-hp Nakajima Ha-117
engine.
The Ki-49-I was first delivered to the JAAF in August 1941, and
the first unit to receive its Donryus was the 61st Sentai in China;
because of the low initial delivery rate, this group kept some of
its older Ki-21s until February 1942. In that same month, the
Donryu made its combat debut, in a raid on Darwin, Australia, on
the 19th. Code-named Helen, it was frequently encountered over New
Guinea and New Britain. But the doubts of the service-test pilots,
alluded to above, were confirmed by actual combat experience. In
addition to its poor handling and lack of power when fully loaded,
the Ki-49’s speed, though superior to the older type, was still not
fast enough to avoid interception, and its effective bomb load was
lower than that of the Ki-21. On the plus side, the Donryu’s crews
thought highly of its superior defensive armament and of its armour
and self-sealing fuel tanks.
In the spring of 1942, it was decided to install a pair of
1,450-hp Nakajima Ha-109 radial engines in the Ki-49 in an attempt
to improve its performance and handling. The oil coolers, formerly
mounted on the front of the older engines, were shifted to scoops
under the cowlings; otherwise, there was little change in the
nacelles as the two kinds of engines were virtually identical in
size. Combat experience dictated further changes, such as a new
bombsight, heavier-grade armour plate, and improved self-sealing
fuel tanks. After testing of two pre-production prototypes, the
revised version was accepted for production as the Ki-49-IIa, and
deliveries commenced in September 1942. However, it was swiftly
realized that rifle-calibre machine guns were not very effective
against Allied fighters, and all of the former 7.7mm guns in the
nose, ventral, and tail positions were replaced by 12.7mm guns in
the major production variant, the Ki-49-IIb.
Despite all the improvements made to it, the Ki-49-II never
totally supplanted the Ki-21-II in service. It was the
best-protected and best-armed JAAF twin-engined bomber until the
Mitsubishi Ki-67 Hiryu was introduced in October 1944, but its
crews were still disappointed at its performance. In particular,
its low and medium-altitude speeds were still deemed
unsatisfactory, and its flight characteristics were not as pleasant
as those of its predecessor. When the Americans returned to the
Philippines in October 1944, the Helen was very heavily engaged;
they suffered massive losses until December of that year when most
of the survivors were expended in suicide attacks against the US
landing force at Mindoro island. Many more were used in the suicide
attack role during the Okinawa campaign; for this mission all the
armament was removed, the crew was reduced to the two pilots only,
and the bomb load increased to 3,527 pounds.
Nakajima was baffled by the type’s continuing problems, and
tried hard to improve the Ki-49; their most ambitious attempt
involved creating a version featuring two examples of the most
powerful fourteen-cylinder radial engine ever devised by any
country, Nakajima’s own 2,420-hp Ha-117. But the Ha-117’s teething
troubles could never be resolved, and only six examples of this
subtype, the Ki-49-III, were built in 1943.
Despite its shortcomings, the Donryu was adapted to perform a
number of additional missions besides “heavy” bomber. Some were
used as troop transports (the great Navy ace Hiroyoshi Nishizawa
was killed while riding as a passenger in a Helen-ironic fate!);
others were field-modified as night fighters. In the night-fighter
role, one aircraft worked as a “hunter” with a searchlight, and a
second was the “killer”, mounting a 75mm cannon, but the Ki-49 made
a disappointing night fighter as it lacked the performance needed
for this role. Still others carried electronic and magnetic
detection gear to act as anti-submarine patrol planes. In the end,
though, the Donryu was a very disappointing aircraft, for all the
effort put into making it an effective warplane. Just 819 examples
of the Ki-49 and its derivatives were built, 769 by Nakajima, and
50 more by Tachikawa Aircraft Ltd., but plans to also produce it in
Harbin, Manchuria, by Mansyu were not realized. Overall production
ceased in December 1944, as the Army had placed its hopes in the
Mitsubishi Ki-67 Hiryu.
Variants
Ki-49
Prototypes and pre-series models with a 708
kW (950 hp) Nakajima Ha-5 KAI or the 1,250 hp Ha-4. The pre-series
with little modifications from the prototype.
Ki-49-I
Army Type 100 Heavy Bomber Model 1, first
production version.
Ki-49-II
Two prototypes fitted with two Nakajima
Ha-109 radial piston engines.
Ki-49-IIa
Army Type 100 Heavy
Bomber Model 2A - Production version with Ha-109 engines and
armament as Model 1.
Ki-49-IIb
Version of Model 2
with 12.7 mm Ho-103 machine gunes replacing rifle calibre
weapons.
Ki-49-III
Six prototypes fitted with two 1,805 kW
(2,420 hp) Nakajima Ha-117 engines.
Ki-58
Escort fighter with Ha-109 engines, 5 x 20 mm
cannon, 3 x 12.7 mm (0.5 in) machine guns. 3 prototypes built.
Ki-80
Specialised pathfinder aircraft - 2
prototypes; employed as engine test-beds.
Total production: all versions 819 examples
(including 50 built by Tachikawa)
Nakajima Ki-49 Donryu (Helen) Technical
Data
Type:
Twin-engined “heavy” bomber, of all-metal construction.
Accommodation:
Crew of eight (two pilots, bombardier, navigator, radio
operator/gunner, and three dedicated gunners) all in enclosed
cockpits, main cabin, or turrets.
Powerplants:
(First prototype) Two Nakajima Ha-5 fourteen-cylinder air-cooled
radial engines, rated at 950 hp for take-off and 1,080 hp at 13,125
ft.
(Pre-production machines and Ki-49-I) Two Nakajima Ha-41
fourteen-cylinder air-cooled radials, rated at 1,250 hp for
take-off and 1,260 hp at 12,140 ft.
(Ki-49-II and Ki-58) Two Nakajima Ha-109 fourteen-cylinder
air-cooled radials, rated at 1,450 hp for take-off and 1,300 hp at
17,330 ft.
(Ki-48-III and Ki-80) Two Nakajima Ha-117 fourteen-cylinder
air-cooled radials, rated at 2,420 hp for take-off and 2,250 hp at
16,075 ft.
Armament:
(Prototypes/pre-production machines, Ki-49-I and Ki-49-IIa) One
flexible 20mm cannon in the dorsal position and one flexible 7.7mm
machine gun in each of the nose, ventral, beam, and tail
positions.
(Ki-49-IIb and Ki-49-III) One flexible 20mm cannon in the dorsal
position, one flexible 12.7mm machine gun in each of the nose,
ventral, and tail positions, and one flexible 7.7mm machine gun in
the port and starboard beam positions.
(Ki-58) Five flexible 20mm cannon and three flexible 12.7mm
machine guns.
Bomb-load:
Normal, 1,653 lbs.; maximum, 2,205 lbs.; suicide attack, 3,527
lbs.
Dimensions, weights, and performance:
Ki-49-I:
Wingspan, 67 ft. 1/8 in.;
length, 55 ft. 1 ¾ in.;
height, 13 ft. 11 5/16 in.;
wing area, 743.245 sq. ft.;
empty weight, 13,382 lb.;
loaded weight, 22,377 lb.;
maximum weight, 23,534 lb.;
wing loading, 30.1 lb./sq. ft.;
power loading, 8.9 lb./hp;
performance figures N/A.
Ki-49-IIa:
Wingspan, 67 ft. 1/8 in.;
length, 54 ft. 1 5/8 in.;
height, 13 ft. 11 5/16 in.;
wing area, 743.245 sq. ft.;
empty weight, 14,396 lb.;
loaded weight, 23,545 lb.;
maximum weight, 25,133 lb.;
wing loading, 31.7 lb./sq. ft.;
power loading, 7.8 lb./hp;
maximum speed, 306 mph at 16,405 ft.;
cruising speed, 217 mph at 9,845 ft.;
climb to 16,405 ft., 13 min. 39 sec.;
service ceiling, 30,510 ft.;
normal range, 1,243 miles;
maximum range, 1,833 miles.
