 Front line units from mid-1940 were equipped with the Ki-21-IIa ("Army Type 97 Heavy Bomber Model 2A") with the more powerful Mitsubishi Ha-101 air-cooled engines and larger horizontal tail surfaces.This became the main version operated by most IJAAF heavy bomber squadrons at the beginning of the Pacific War, and played a major role in many early campaigns.For operations over the Philippines the JAAF's 5th, 14th and 62nd Air Groups, based in Taiwan, attacked American targets at Aparri, Tuguegarao, Vigan and other targets in Luzon on 8 December 1941.