Nonetheless, the Ki-44 was the fastest climbing Japanese fighter at the time. It was the Imperial Japanese Army's only interceptor type when the USAAF's B-29 Superfortresses began bombing the Japanese mainland in June 1944. While there was performance restrictions at high altitude, it was superior to the Ki-43 in that it was capable of matching Allied aircraft in climbs and dives, giving pilots more flexibility in combat and greater pilot confidence than the Ki-43; the basic armament of four 12.7mm machine guns or two 12.7mm guns and two 20 mm cannons (or, in a few aircraft, two Ho-301 40mm cannons of limited range) was far more powerful than the older Ki-43's two 12.7mm machine guns.