Staff Writer (Updated: 8/24/2015):
Initially, the Ki-109 program was to utilize two Ki-109 aircraft working in conjunction with one version fitted with a searchlight and powerful radar while the other operated with a powerful large caliber weapon. This "hunter-killer" pairing was to work in unison at detecting incoming enemy threats and eliminating them in total darkness. The system, however, proved to be quite complicated and thusly only a single Ki-109 design would emerge, now as a day interceptor, though still mounting a large caliber weapon.
The Ki-109 retained many of the visual qualities of the Ki-67 heavy bomber. Among the chief changes were the lack of much defensive armament and the use of more powerful engines over her predecessor (2 x Mitsubishi Ha-104 series piston engines at 1,900 horsepower). The Ki-109 was defensed by only a single 12.7mm heavy caliber machine gun in the tail. Beyond that, her armament was all-offensive and consisted of a 75mm anti-aircraft cannon in the nose. This system was manually loaded but could be fired with the Ki-109 aircraft still well out of the range of the B-29's defensive machine gun armament.
The new Mitsubishi Ki-109 entered limited production that eventually totaled only some 24 total examples (2 were the day version prototypes). Though maintaining the maneuverability of the Ki-67, the system still proved to lack true high altitude performance that was required of the type. This was basically moot by this point in the war, however, as the American bombers were now conducting low-level nighttime operations for improved accuracy. The Ki-109 - at best - was a stop-gap design that never saw its true operational potential fulfilled. But such was the case when operating from a completely defensive standpoint as the Empire of Japan was by this point in the conflict.