https://ja.wikipedia.org/wiki/%E4%BA%8C%E5%8D%81%E8%A9%A6%E7%94%B2%E6%88%A6%E9%97%98%E6%A9%9F
Semi auto translation. ;D
"20-shi Ko fighter was the fighter that the IJA planned.
There is a possibility that the name “20-shi Ko fighter” isn't formal, but this is the name which is widely used up to now.
The IJA considered "high altitude Ko fighter" as a plan policy of a new fighter and made public in each manufacturer by a joint session of officials and the aircraft makers in 1945 May 1.
I this session, following three designs were discussed.
(1) Modified version of Reppu or Shiden-kai.
(2) Jinpu.
(3) New design.
Then selected Reppu engine exchange type and Shidei-kai engine exchange type as "decisive battle fighter".
And decided to develop new plan “next fighter”. Later this next fighter became “20-shi Ko fighter”.
Mitsubishi "HA 43-51", Nakajima "HA 44-21" and Nakajima "HA 45-44" engine were designated as an engine of "the next fighter". The following target value had been submitted from Yokosuka navy flying corps.
* Maximum velocity : 704 km/h ( 10,000 m),landing speed, below 148 km/h.
* Time to reach 10,000 m : Within 15 minutes. Service ceiling : 13,500 m.
* Endurance : cruises, 2.5 hours + full power 0.7 hours.
* Armament : four 20mm cannon (250 bullets) .
* Fuselage fuel tank : Rubber shellproof.
*Equipped with automatic extinguish device in a wing.
*Have defense.behind the pilot, magazine upper surface and back surface .
Mitsubishi Heavy Industries, Kawanishi and the first arsenal of munitions ( which Nakajima Aircraft changed to this name. Below called as "Nakajima") have submitted the performance estimation value in joint session of officials and the aircraft makers in May 23 1945 according to this request.
Mitsubishi submitted three kinds of plans for which Reppu performance improvement type, Reppu-kai modification type and perfect new design plans.
Kawanishi submitted the plan which Shiden-kai modification type, and Nakajima perhaps submitted the plan which Ki 87 modification type or Tachikawa Ki 94II base design..
An improvement plan of Reppu-kai was selected as "the next fighter" from these plans , and decided to develop this fighter by Mitsubishi and Nakajima joint work, because expected install "HA 44-21" engine was Nakajima’s design.
This fighter was 20-shi Ko fighter. In June 10 1945, required performance was decided.
After that “HA 44-13"engine which had high achievement possibility was added as the candidate for alternate engine.
The IJA expected that the operational date of 20-shi Ko fighter was beginning in 1947, but development was canceled at the plan stage with the end of the war.
20-shi Ko fighter was the IJA planned last fighter.
Final 20-shi Ko fighter plan was based on Reppu-kai or Reppu type 11, all data except payload were near target value.20-shi Ko fighter difference compared with Reppu were engine”Ha44-21” and larger weight propeller.
The cruising range, the armament and the shellproof etc. are equal to the Reppu performance improvement type.
An automatic air combat flap was planning to be improved to secure the air combat performance in the high altitude.
20-shi Ko-fighter specification (the planned value)
Span : 14M, Wing area : 31.3 m2, Empty weight : :3,392 kg, Gross weight : :5,753 kg, Engine : Nakajima HA 44-21 18 cylinders of air-cooled radial (2200 horsepower)x 1, Maximum speed : :657 km/h, Cruising speed : :417 km/h, Service seiling : :10,500 m, Endurance : cruise for 2.5 hours+ full speed for 0.6 hours, * Armament : four type-99 20-go 5-gata 20mm cannon.
It was written on joint work "zero fighter" of Jiro Horikoshi and Okuyama MasatakerShobu, but there were no names as "20-shi Ko fighter" in official documents in those days, and after beginning to plan, the name as "next fighter" was called.
It was said that a nickname as "land breeze" was scheduled."
Ko-fighter(甲戦闘機) : Air superiority fighter, single seater.(A real fighter.)
Otsu-fighter(乙戦闘機) : High altitude interceptor, single seater.
Hei-fighter(丙戦闘機) : Night interceptor, twin seater.
Speculative Kawanishi plan.
http://www.warbirds.jp/kakuki/kaksasie/hon/N1K2-J2.html
HA44-21 ENGINE : two stage three speed mechanical supercharged engine.