Mitsubishi Ki-21, part two

 

Losses for the Ki-21 were high during early combat operations due to an insufficient defensive armament - the Ki-21-Ia was armed with three 7.7mm Type 89 machine guns in nose, ventral and dorsal positions - and self-sealing fuel tanks, with the aircraft's fuel system proving to be unreliable. To address those problems, the Ki-21-Ib was introduced. An additional defensive machine gun was introduced in the tail in an 'stinger' arrangement and was remotely controlled. Both horizontal tail surfaces and trailing edge flaps were improved and the bomb bay was enlarged. Fuel tanks were also partially protected with laminated rubber sheets. 

A third variant of the Ki-21-I was introduced in 1940. This variant had an additional 500 L (130 US Gal) auxiliary fuel tank, fitted in the rear weapons bay and two more machine gun placed in the beam position, making a total of six. Underwing hardpoints were also fitted to carry a total of four 50 Kg (110 lb) bombs externally and, in order to compensate for the increase in weight, the wheels were increased in size. 

By the time of the attack on Pearl Harbor and the start of the Pacific War, the Imperial Japanese Army Air Force's (IJAAF) main antagonist until then, the Republic of China Air Force (ROCAF) had been greatly improved to the point that the Ki-21-I's losses were mounting too high, so most Ki-21-Ia and some 21-Ib were relegated to second line and training duties. 

Front line units were equipped, from mid 1940 onwards with the Ki-21-IIa (Army Type 97 Heavy Bomber Model 2A). This variant was powered by the much powerful Mitsubishi Ha-101 radial engine which yielded up to 1.500 hp of power and was also equipped with larger horizontal tail surfaces. This variant was the standard for many IJAAF's heavy bomber squadrons at the beginning of the Pacific War and played a major role in the early campaigns of the War. For operations over the Philippines, IJAAF's 5th, 14th and 62nd Sentais (a Japanese unit equivalent to a Wing or Group), based in Formosa, attacked American targets at Aparri, Tuguegarao, Vigan and other targets in Luzon island on 8th December 1941. The 3rd, 12th, 60th and 98th Sentais, based in French Indochina bombed British and Australian positions in Thailand and Malaya attacking Alor Star, Sungai Petani and Butterworth (Penang) escorted by Nakajima Ki-27 and Ki-43 fighters. However, during those operations, the Ki-21 suffered heavy losses both from the Hawker Hurricane and the Curtiss P-40 Warhawk.

Sources:

1. https://en.wikipedia.org/wiki/Mitsubishi_Ki-21

2. https://www.valka.cz/Micubisi-Ki-21-Sally-t31499 (translated)

3. http://www.asisbiz.com/il2/Ki-21-Sally/Mitsubishi-Ki-21-Sally.html