The Mitsubishi Ki-21 (Allied codename: Sally) was a Japanese bomber during World War II. It began operations during the Second Sino-Japanese War participating in the Nomonhan Incident, and in the first stages of the Pacific War, including the Malayan, Burmese, Dutch East Indies and New Guinea Campaigns. It was also used to attack targets as far-flung as western China, India and northern Australia. Variants of the Ki-21 medium bomber remained in production until 1944 and were in service until the end of the Second World War.
The Ki-21 had more then one Allied codename. Initially called Jane, the name was changed to Sally. When the Ki-21-IIb variant entered service, Allied observers to mistakenly believed it for a completely new type, which was designated Gwen. When it was realized that Gwen was a new version of the Ki-21, it was renamed Sally 3, with Sally 1 referring to the earlier Ha-5 powered models, and Sally 2 referring to the Ha-101 powered Ki-21-IIa.