Extra info from the Japanese Wiki:
Nakajima Aircraft proceeded with the design and development under Chief Engineer Matsumura Kenichi, and in October 1944, the first prototype was completed and successfully made its maiden flight on 23 October. However, due to a shortage of aluminum, the first prototype was unable to achieve the planned performance. Furthermore, while being moved to the hangar, a brake failure caused a collision, damaging the nose. As the turbosuperchargers were also incomplete, delivery to the Navy was delayed until January 1945.
Prototypes 1 to 3 were primarily used to verify the condition of the airframe and its fittings; final performance tests using aircraft fully equipped with turbosuperchargers and armament were scheduled to begin with Prototype 4 onwards. Although high performance was anticipated, due to accidents and air raids, and the cancellation of production of the aircraft itself, sufficient test flights were not carried out, and its actual performance remains unclear.
Although the "Renzan" Prototypes 1 and 2 were accepted by the Navy and began trials based at Oppama Airfield, Prototype 1 suffered an accident during a test flight landing in which the rear fuselage snapped. Following an investigation, it was found that the fuselage strength specifications in use at the time were inadequate for aircraft with a nose-wheel landing gear configuration; consequently, repairs were carried out alongside a redesign.
As Oppama Airfield was cramped and unsuitable for testing heavy aircraft, and as US air raids on the Kanto region had intensified since February of that year, Prototypes 1 and 2 of the "Renzan" were airlifted to Misawa Naval Air Station in March to avoid these attacks. Flight tests aimed at overloading the aircraft were conducted there; however, due to the worsening war situation, the "Renzan" prototype program itself was canceled in June 1945. Tests using the turbosuperchargers were never carried out, and the aircraft were destroyed in an air raid at the same location in August. Nakajima Aircraft had halted work at the stage of manufacturing the fuselages for prototypes 5-8 at Koizumi Works, leaving a total of two aircraft-prototypes 3 and 4, which had been completed but not yet accepted-at the adjacent Koizumi Airfield. One of these was severely damaged by air raids, while the other remained in a partially damaged state.
This damaged airframe (believed to be Prototype No. 4, though there are differing theories) was seized by the US military. After repairs were carried out using salvaged parts from Prototypes No. 1 and No. 2, which had been transported from Misawa, it was airlifted from Koizumi Airfield to Oppama in December 1945 under US military supervision. It was then loaded onto a ship at Yokosuka and transported to the United States.
In June 1946, the transferred aircraft flew the 480-nautical-mile route from Newark Army Airfield to Patterson Airfield in Ohio at a speed of 110 miles per hour, covering the distance in 3 hours and 10 minutes against a headwind of 26 knots. Although one more test flight was conducted, subsequent flights were canceled due to the poor condition of the aircraft; for instance, even after refurbishment by the US military, only one of the four engines was capable of operating at full power. The aircraft was stored on the base as a candidate for display in the Air Force Museum, but was ultimately scrapped during the Korean War, and no examples survive today.
This photo and the next two, feature Prototype Number 2 found in Misawa base, Aomori Prefecture. It suffered serious damage on the fuselage, as can be seen in the photos, when it was strafed during an air raid.
I’d like to conclude these posts on the "Renzan" by sharing my thoughts on the aircraft.
The "Renzan" was supposed to be the replacement Navy bomber of the Mitsubishi G4M "Betty". If everything had gone well with the engines, it would be faster than the "Betty"; "Rita" max speed 320kt, "Betty" 254kt.
"Rita" would also have significantly increased range; "Rita" 2130 naut miles, max 4030, "Betty" 3270 or 2340.
"Rita" would also have drastically improved defensive armament with the rotating dorsal and ventral turrets and the twin 20mm cannons on the nose and tail.
Other than that...nothing else. In my eyes Nakajima, again, strived to improve the Mitsubishi design by trying to correct some of its weaknesses but did nothing to improve the payload.
I'll keep it simple to be easily understood.
If an ordnance of 100 tonnes is necessary to damage an enemy airfield significantly enough so that it remains out of comission for a meaningful amount of time (one week), then 20 bombers carrying a payload of 5 tonnes (ex. Consolidated B-24 Liberator) would need to fly 3-4 missions.
Now consider how many missions or how many aircraft would be needed if you have bombers that can only carry a one-ton payload. In fact, Japanese bomber crews frequently expressed frustration that any damage they caused to the enemy airfield during night missions was gone by the following morning. Certainly, credit goes to the Seabees, but this also clearly illustrates the critical shortcomings of the Japanese bombers.
Therefore, if the "Renzan" was not intended for strategic bombing missions but rather to function more as an attack bomber, akin to the "Betty," then Mitsubishi had already completed and started production of one the finest attack bombers: the Ki-67 "Hiryu" (Peggy).
All in all, I fail to see how the "Renzan" would be of any use to the Japanese Navy. A testbed for some improvements like the turrets and the turbosuperchargers, okay, but nothing more.