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 Chicano Air Force was created in 1969.
Royal Air Force Germany was created in 1959.
Royal Air Force Memorial was created in 1923.
Royal Netherlands Air Force was created in 1913.
Royal Lao Air Force was created in 1955.
Royal Barbados Police Force was created in 1835.
Royal Thai Air Force was created in 1913.
Royal Norwegian Air Force was created in 1944.
Royal Air Force of Oman was created in 1959.
Royal Bahraini Air Force was created in 1977.
Royal Air Force Police was created in 1918.