No. 21 Squadron is a Royal Australian Air Force (RAAF) general reserve squadron. Formed in the mid-1930s as a unit of the part-time Citizen Air Force (CAF), it was mobilised for service during World War II, when it saw action against the Japanese as a fighter unit in the Malayan campaign, a dive bomber unit in the New Guinea campaign, and a heavy bomber unit in the Borneo campaign. After the war, the squadron continued to fly until 1960, when the CAF ceased flying operations. At that time, No. 21 Squadron converted to a non-flying support role, which it currently fulfils at RAAF Williams.