The Air Force originially started as part of the Army (Army Air Corp) during WWII. It became a separate branch after the end of WWII.
Womens Army Corp
The US Army, like the US Department of Defense, is supported by the tax payer. The US Army Corp of Engineers is part of the US Army; just like the US Army Artillery, or US Army Armor, or US Army Infantry.
Military branch (Army, Navy, Air Force, Marine Corp) Psychological Services.
Air force
The U.S. Air Force was once the Army Air Corps.
Yes, Canadians served in the navy, Royal Flying Corp and army.
Yes.
The RAF was formed on 1st April 1918 (Yep...No April fools Joke) by the amalgamation of the royal flying corp (Army air corp) and the Naval air service. It is the youngest and by far the most civillised armed service, commonly called the 'gentlemans armed service' as apposed to the army and navy where the conscrips of criminals and vagabonds were commonly 'called' upon to fill the ranks.
The Army initiated the Army Air Corps which changed into the United States Air Force after World War II. Viper1
Womens Army Corp
Army and Marine corp can give 'moral waivers' and allow a person with only 1 felony conviction to enlist. If more than 1 conviction you are SOL for army or marine corp. The air Force and Navy allow waivers for multiple convictions, but waivers are a lot harder to get in these branches of the service.