Answer
Several countries began their "air force" as a small part of the Army since no one thought airplanes would be of any value. In the US, the first pilots were organized under the branch of the Signal Corps. Later, they were separated into the Army Air Corps but still under the command of the Army. It wasn't until 1947 that the US Air Force became a separate branch.
The British were ahead of the US in creating a separate branch for the Air Force. Other countries also had their air force in a separate branch.
Royal Naval Force was created in 1920-10.
Royal Malaysian Air Force was created in 1958.
Royal Brunei Air Force was created in 1991.
Royal Norwegian Air Force was created in 1944.
Royal Air Force Memorial was created in 1923.
Royal Danish Air Force was created in 1950.
Royal Air Force Commandos was created in 1940.
Royal Air Force of Oman was created in 1959.
Royal Bahraini Air Force was created in 1977.
Royal Air Force Germany was created in 1959.
Royal Cambodian Air Force was created in 1953.
Royal Netherlands Air Force was created in 1913.