In summer 1935, the Imperial Japanese Navy Air Service (IJNAS) issued a specification, 10-shi, for a carrier-based strike-bomber with torpedo capabilities. The specification called for a maximum speed of 330 km/h (205 mph) , an endurance of up to 7 hours unloaded and 4 loaded with a single torpedo plus folding wings to facilitate its usage in a carrier. Those requirements exceeded by far the performance numbers of the winner of the previous specification, 9-shi, the Yokosuka B4Y. Regarding armament, the 10-shi called for a total payload of up to 800 kg (1.764 pounds) plus a rear-defensive machine-gun. The resulting machine had to be powered by the Mitsubishi MK8 Kinsei radial engine.
Two old rivals, Mitsubishi and Nakajima (which eventually won the contest with the famous Nakajima B5N), were called to participate in the contest, with Yokosuka not taking part although they won the previous contest.
Mitsubishi's project for the 10-shi, received the internal designation of Ki-16 and was designed by Jiro Horikoshi, designer of the successful A5M and the more successful A6M Zero. Inspired by the Northrop 5A (a prototype for the Northrop YA-13 which was sold to the IJNAS by the USA), Horikoshi decided to adopt a fixed landing gear, relying on the success of the A5M, covering wheels with a massive cowling. He also refused the mechanical wing-folding drive the 5A had in order to not overload the engine with additional tasks.
The B5M prototype was completed in January 1937, almost at the same time the B5N's prototype was completed. Both bombers were tested and the B5M showed a slower speed than the B5N, 355 km/h (221 mph) versus 368 km/h (229 mph) of the B5N.
During the year 1937, both airplanes were tested, with the IJNAS' high command divided between the two. On one hand they liked the good flying characteristics of the B5N, but on the other, they saw it as a very progressive machine, as it had many new characteristics for naval bombers of the time, such as retractable landing gear. So, in late 1937, they IJNAS' high command opted to adopt both designs under the same denomination, of "Type 97 Deck Attack Aircraft of the Fleet". To distinguish both bombers, "Type 97 Model 1" was the B5M and "Type 97 Model 2" was the B5N.
Unfortunately for Mitsubishi, Nakajima was able to fix the problems of the earlier models of the B5N, while Mitsubishi didn't as they were more concentrated on improving the A5M and designing the A6M, therefore, the B5M was discontinued with only 125 machines having been built between 1936 and early 1941.
The B5M1 (an improved model with a new engine, from 1939) was powered by the Mitsubishi MK8D Kinsei-43 radial engine which, in its newest models, had a theoretical power of 1.080 hp, raising this way its top speed to 381 km/h (234 mph), making it faster than the B5N. However, the IJNAS still decided in favour of the B5N.
The first combat unit to be equipped with the B5M in late 1938 was the newly formed Suzuka Kokutai, based in Eastern Japan. This unit undertook the task of training and evaluating the B5M.
Contrary to popular belief, the B5M was not deployed to China, as every machine was deployed in combat training units in mainland Japan and, in early 1941, they were assigned as follows:
- Oita Kokutai: 45 units
- Usa Kokutai: 63 units
- Omura Kokutai: 16 units
All those units were deployed at Kure Naval Base, in Tokyo and carried out anti-shipping patrols over the Pacific Ocean, after Pearl Harbour.
As the IJNAS was in need of every aircraft they could count on, the B5M was rushed to many front-line units, so many B5Ms were assigned in early 1942 to the 33rd Kokutai which took part in the battle of Celebes Island, in the Dutch East Indies. It is not known the exact number of machines that served with the 33rd Kokutai, but by the time that unit was re-organized as the 932th Kokutai, in the summer of 1943, there were still many B5Ms active.
One of them became famous when, at the hand of Captain Kino, rammed the leader of a formation of American B-24 bombers that bombed the city of Makassar.
From July to October 1942 some B5Ms were assigned to the escort-carrier Un'yo, serving in the area of Truk Atol. It seems too that some machines were retrofitted with forward-firing machine guns, however, as the war went on, the usage of both the B5M and the B5N decreased and, apart from a sporadic anti-submarine patrol, the B5M wasn't used.
The last known usage in combat of a B5M was during the Kamikaze attacks in Okinawa during April-May 1945.
Sources:
1. http://www.airwar.ru/enc/bww2/b5m.html (translated)
2. https://en.wikipedia.org/wiki/Mitsubishi_B5M
3. https://www.skytamer.com/Northrop_1933(Gamma5D).html (for the Northrop 5A)
4. https://www.valka.cz/Micubisi-B5M1-Mabel-t30172
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