The Mitsubishi Ki-21 was the Imperial Japanese Army's primary long-range bomber at the start of World War II.
The Ki-21 was robust enough against the weakly armed Chinese fighters to hold its own until the early 1940s. When the Americans appeared in the Pacific with more heavily armed and significantly faster fighters, the Ki-21's losses increased enormously despite constantly improved armament and armor. Even escort protection in the form of fighters was of little help, as these generally had shorter ranges, so that the Ki-21 was completely outdated in its role as a long-range bomber by 1942.
Source: Wikipedia
