Many thanks Steve san, redstar72 san and T-50 san!
In May of 1943, the IJA decided to cancel Ki-85(modified Shinzan,G5N,), ordered Kawasaki to develop long range heavy bomber Ki-91 in 9th of July.
Required Ki-91 performance were as follows.
Maximum speed : 580km/h(at 10,000m)
Range : 9,000km with 4 ton bomb
Armament : 20mm cannon ×9, bomb 8ton(max)
To realize required performance, pressurized cabin and turbo charged engine were needed.
But Kawasaki did not have pressurized cabin technology at the day, decided to develop No.1 aircraft without pressurized cabin and used as a stability and control test bed. Finally No.1 aircraft was planned to have pressurized cabin.So I believe that the planned final shape of Ki-91 was little different from this 3 side view drawing and this wind tunnel test model shape, especially cabin shape. We have the chance to guess it's shape . No.1 aircraft was planned to complete in June 1946, No.2 aircraft was planned to complete in march 1947. The first full scale mock up examination was carried out in April 1944, the second was carried out in May 1944. In June 1944, fabrication of jig begun. But in November 1944, B-29 bombing became hard, it became impossible to continue development for such a big aircraft, Ki-91 was canceled in February 1945 when 60% of design was finished. Wing cross section was laminar flow type which applied Ki-94 fighter. Finally HA-50 engine was planned to use.(same as Fugaku)
I think Ki-91's mission was Saipan B-29 base bombing.
Source : All the Experimental Aircraft in Japanese Army, Kantosha, ISBN978-4-87357-233-8 C9476