 Losses were high during early combat operations, with weaknesses including a lack of armament and self-sealing fuel tanks, while the aircraft's oxygen system also proved unreliable.The Ki-21-Ib was an improved version designed to address the armament issue by increasing the number of 7.7 mm (.303 in) Type 89 machine guns to five, and incorporating improvements to the horizontal tail surfaces and trailing edge flaps.In addition, the bomb bay was enlarged.