Nakajima Ki 49 Helen

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.

Natural-metal Ki-49s in formation over Japan.

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.

A Ki-49-I of the Hamamatsu Bomber School taking off.

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.

A Ki-49-IIb abandoned in the Philippines, 1945.

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.

Close-up of a Ki-49-IIb warming up.

Nakajima Ki-49 Donryu (Helen) Technical Data

Twin-engined “heavy” bomber, of all-metal construction.

Crew of eight (two pilots, bombardier, navigator, radio operator/gunner, and three dedicated gunners) all in enclosed cockpits, main cabin, or turrets.

(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.

(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.

Normal, 1,653 lbs.; maximum, 2,205 lbs.; suicide attack, 3,527 lbs.

Dimensions, weights, and performance:

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.

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.