 The first prototype flew on 18 December 1936, with the second prototype, which differed in replacing the dorsal turret of the first prototype with a long greenhouse canopy, following later in the month.In the resulting competition Mitsubishi's Ki-21 and Nakajima's Ki-19 were found to be similar, with the Ki-21 having better performance while the Nakajima design was a better bombing platform and had more reliable engines.In order to make a final decision, two further prototype were ordered from both Mitsubishi and Nakajima, with Mitsubishi instructed to change its own Mitsubishi Ha-6 radial engines for the Nakajima Ha-5 engines used by the Nakajima design and vice versa, while the Ki-21 gained a revised glazed nose similar to that of the Ki-19 and revised tail surfaces.