Let's begin with what Francillon has to say about the Kawasaki Ki-60 & the Ki-61 "Hien"
Ki-60:
"While negotiating with Daimler-Benz A.G. for the manufacturing rights for the German DB 601A inverted-vee liquid-cooled engine, Kawasaki were successful in impressing upon members of the Koku Hombu that most contemporary foreign high-performance fighters were powered by liquid-cooled engines. Consequently, in February 1940, Kawasaki were instructed to design around the German engine, or its Japanese version, two fighter aircraft: the Ki-60 heavy interceptor and the Ki-61, a lighter all-purpose fighter. "
Ki-61:
"While negotiating with Daimler-Benz, Kawasaki had approached the Army with initial design studies for various fighter aircraft making use of this engine. As reports from the air war in Europe were showing the apparent superiority of aircraft powered by liquid-cooled engines, the Koku Hombu instructed Kawasaki in February 1940 to proceed with two aircraft of this type: the Ki-60, a heavy interceptor, and the Ki-61, a lighter all-purpose fighter, priority being given to the heavier aircraft".
The Japanese Wikipedia and virtually all Japanese sources see things differently:
"In the summer of 1939, when the Nakajima Ki-43 "Hayabusa" [Oscar], was still undergoing evaluation, the Army began preparations for the development of a new fighter aircraft, exchanging opinions and conducting inspections with various companies. While air-cooled radial engines manufactured by Nakajima and Mitsubishi dominated the Japanese aviation engine landscape, Kawasaki had—since acquiring the manufacturing rights for the German BMW liquid-cooled V-type engine in 1924—developed, produced, and installed that engine (and its improved variants) in its own aircraft. However, the engine series culminating in the Ha-9 model had reached its performance limits, necessitating a shift to a new type. Meanwhile, liquid-cooled engines remained highly valued overseas for high-speed military aircraft; unable to abandon this technology, the Japanese military decided to domesticate the German Daimler-Benz DB601 engine. In 1937, the Navy assigned the acquisition of manufacturing rights to Aichi, while the Army assigned them to Kawasaki—both companies having prior experience in liquid-cooled engine production. Kawasaki, slightly lagging behind Aichi, dispatched a team of engineers to Germany in 1939 and invited technicians from Daimler to assist, ultimately completing the first domestically produced DB601A in December 1940. After further research, Kawasaki submitted plans for the light fighter KDA-21 and the heavy fighter KDA-22 in January 1940.
Under the Japanese Army's "Research Policy" and "Research Schedule for Next-Generation Aircraft" dated 1940, the goal was to first complete a "Type 1" interim model—equipped with two 20mm cannons and capable of a top speed in the 550 km/h range—by December 1940, followed by the completion of evaluations for this interim model by September 1941, with the development of the final operational version to proceed based on those results.
As this was essentially Japan's first attempt at a dedicated heavy fighter, the research schedule for future aircraft outlined a phased approach: first, an interim model armed with two 20mm cannons would be completed by December 1940 and undergo evaluation by September 1941; subsequently, a production-ready model would be developed based on those results. The target maximum speed for the final operational version was envisioned to be in the 640 km/h range.
In February 1940, acting on this policy, directives were issued for the development of the Ki-60 heavy single-seat fighter powered by the DB601A and the Ki-61 light single-seat fighter powered by the Ha-40 (the domestic version of the DB601A) as the next generation of fighters. The chief engineer for both was engineer Takeo Doi.
The first Kawasaki Ki-60 prototype.
The type incorporated various new technologies, such as a sliding canopy designed for improved rearward visibility, a retractable tailwheel, and—for the first time in Japan—retractable main landing gear that shortened in length during the retraction process. This shortening-retraction mechanism was adopted despite the resulting mechanical complexity and increased weight in order to narrow the spacing between the wing-mounted cannons.
The English Wikipedia has this about the Ki-60:
"From the start of flight testing, it became apparent that the design was seriously flawed in several key areas. The take-off run was unacceptably long, while, in flight, the aircraft displayed some lateral instability, excessively heavy controls, and poor control response. The spinning characteristics were described as "dangerous", and the stalling speed was extremely high. Although a top speed of 600 kilometres per hour (370 mph) had been projected the Ki-60 was only able to achieve 548 kilometres per hour (341 mph)."
The source is mentioned as "Green, William; Swanborough, Gordon (1976). JWW2 Aircraft Fact Files: Japanese Army Fighters, Part 1."
Curiously, absolutely no Japanese source mentions anything even remotely close to the above statements.
On the contrary, Japanese sources mention that:
"Test results confirmed that the aircraft achieved a maximum speed of 560 km/h at an altitude of 4500 m, a climb time of 6 minutes to 5000 m, and a service ceiling of 10000 m. This was about 15 km/h faster in maximum speed and 30 seconds faster in climb time compared to the Nakajima Ki-44 "Shoki", which had been completed and was undergoing testing earlier."
"In June 1941, during the height of the evaluation process, a comparative test was held in Kakamigahara between a Messerschmitt Bf 109E imported from Germany and Japanese fighter planes. The Ki-60 also participated in this test, and it was found that it could engage in aerial combat on equal or slightly superior terms with the Bf 109E.
Compared to its rival, the Nakajima Ki-44 "Shoki", the Ki-60 had superior turning performance as long as the Ki-44 didn't use its combat flaps, making it easier to pilot and take off/land than the Ki-44. The pilot of the Ki-60 at that time was Captain Kuroe Yasuhiko, who became captivated by the Ki-60's charm and a fan of it ever since."
Other Ki-60 details missing from all non-Japanese sources:
a. "the Ki-60 was armed with 20mm cannons and designed strictly for "hit-and-run" tactics."
b. "The engines installed in the Ki-60 were units imported from Germany to serve as samples for domestic production."
dc. "A total of three prototype aircraft were produced. Two of these were deployed to the 47th Independent Air Squadron alongside pre-production models of the Ki-44, but both were lost in accidents before they could see combat. The remaining aircraft survived until the end of the war."
Finally, completely contrary to what all non-Japanese sources say about the type, all Japanese sources agree that:
"Although the aircraft was ultimately not adopted for service, flight test results indicated it was an excellent machine that fully met its design objectives."
The main reason for the Army rejecting the Ki-60 is that the Ki-61 "Hien", demonstrated unexpectedly outstanding performance.
As for the "Hien", here's a piece from the Japanese Wikipedia, that includes details largely absent from non-Japanese references:
"The design concept for the Ki-61 was that of a versatile fighter—sometimes referred to as a "medium fighter"—that did not strictly adhere to the "heavy fighter" or "light fighter" categories defined in the "Aviation Weaponry Research Policy." It is said that internal Army debate inevitably arose because the requirements for light single-seat fighters demanded not only maneuverability and climbing ability but also the installation of 12.7 mm cannons. Deputy Chief Engineer Owada stated that "a fighter aircraft must surpass the enemy in overall performance; categorizing them into 'light' and 'heavy' fighters is irrational", a view that was shared by the entire Kawasaki development team. Some sources even indicate that the development team referred to the concept as a "medium fighter", suggesting the idea originated with Kawasaki.
Doi himself stated that he sought to craft the Ki-61 into an ideal fighter, without being bound by the Army's "light fighter" doctrine. Detailed design work began around September 1940.
Additionally, Watanabe Yoji in his "The Liquid-Cooled Fighter “Hien”: The Silver Wings of a Japanese-German Collaboration" notes that as early as May 1940—during the initial development phase—Doi had already discussed the possibility of equipping the Ki-61 with an air-cooled engine.
The wooden mockup review took place in June 1941 [23], and the prototype was completed and made its maiden flight in December 1941. Although some sources suggest that the Army had low expectations for the Ki-61—given that it utilized an engine of the same family as the Ki-60—it achieved a top speed of 590 km/h during testing, surpassing both the Ki-60 and the Bf 109E by 30 km/h. This level of performance was entirely unexpected, even by its designer, Doi. For context, other Army fighters of the era had lower top speeds: the "Hayabusa" (a "light fighter") ranged from 495 km/h to 515 km/h, while the "Shoki" (a "heavy fighter", prior to official adoption) topped out at 580 km/h. Consequently, Doi and Owada were awarded the Mainichi Aviation Prize in October 1942 and the Army Technical Merit Award in December 1943.
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