First introduced in a 1,000 PS prototype in 1933, about 7,000 civilian and 5,500 military Ha-5's were built during World War II.
In 1917, Chikuhei Nakajima set up the "Airplane Institute" at Ojima Town in Gunma Prefecture.
[1] In 1920 the company sent Kimihei Nakajima to France to study European advances, and in 1922 started their own engine factory in Tokyo.
He observed the English Gloster Gamecock fighter with its Bristol Jupiter engine, which was an advanced design for the era with an automatic adjustment device for tappet clearance, spiral intake piping for even charge air distribution, and a four-valve intake and exhaust system.
In 1929, the "AH" design, with bore and stroke of 146 × 160 mm and a total displacement of 24.1 L, was completed.
[1] In June 1930 the first prototype of was completed and it passed the durability test for the type approval in the summer.
Nakajima had successfully designed the first original Japanese air-cooled 9-cylinder engine, the 450 PS "Kotobuki", which would over time be developed into the Ha-5 and its successors.
[1] Nakajima knew that engines of higher power would be needed and began work on the Ha-5, a new 14-cylinder, twin-row design that was based on the 160 × 180 mm cylinder of the Hikari.
According to two surviving IJAAF mechanics in 1987, the Ki-84N had a recorded speed of 448–450 mph at a ceiling of 42,254 ft when using war emergency power.