The Daimler-Benz DB 601Aa was a development of the earlier DB 600, with direct fuel injection replacing the carburetor. Like all DB 601s, it had a 33.9 litre displacement. The first prototype with the direct fuel injection was test run in 1935, and an order for 150 engines was placed in February 1937.
A manufacturing license was granted to Aichi, for production of this engine for the IJN as the Atsuta and to Kawasaki for production of this engine for the IJAAF as the Ha40. Under the 1944 Unified System, this engine was re-designated as the Kawasaki Ha-60.
A new high-horsepower narrow profile engine was required for the Kawasaki Ki-64 experimental fighter. The aircraft design called for a narrow profile fuselage, and the solution that Kawasaki developed was the Ha-201 engine. Although similar to the Aichi Ha-70, where two Aichi Atsuta engines, mounted side-by-side behind the cockpit, drove a single propeller, this arrangement called for the two Kawasaki Ha-40 engines to be separately mounted, one in the aircraft's nose, the other behind the cockpit.
The engines were connected to a common gearbox that was mounted in the nose. The rear engine was connected to the nose mounted gearbox by long a drive shaft similar to the American Bell P-39 Airacobra. The gearbox did not combine the power output of the two engines. Instead, the rear engine drove the forward controllable pitch propeller, while the front engine independently drove the rearward fixed pitch propeller.
Up to 1,175 PS (864 kW) at sea-level with 2,500 rpm, up to 1,100 PS (809 kW) at 2,400 rpm and 3.7 km altitude. Used in the Kawasaki Ki-61.
Unified designation for the Ha40
An up-rated 1,500 hp development of the Ha40 for high altitude Kawasaki Ki-61-II KAI interceptor aircraft.
Two Ha40 coupled together with a common gearbox, driving a twin three-blade contra-rotating propeller. Used in the Kawasaki Ki-64. The combination was rated at 2,350 PS (1,728 kW) at sea-level and 2,500 rpm, up to 2,200 PS (1,618 kW) at 2,400 rpm and 3.7 km altitude.