How We Got the USAF

Did the stork bring it? Sorry, but no.
The National Security Act of 1947 created the Air Force. As part of the politicking that followed its passage, the so-called “Key West Agreement” was struck: the Army could have helicopters, and fixed-wing aircraft of less than (IIRC) 12,000lbs, and that was it. The Army wasn’t supposed to arm any of their fixed-wing aircraft, either, although eventually that restriction was de facto relaxed. That’s how the Army ended up with cool planes like the OA-37 Dragonfly and the OV-10 Bronco.
As time passed, the Air Force began to lose interest in the boring close-air support mission, instead spending more of its time and budget on strategic deterrence and air superiority. Over time, this focus has changed somewhat, with air mobility becoming (rightfully) more important than in the past. However, the fact remains that the Army and the Air Force would probably both be happier if the Army got the fixed-wing CAS assets and formed an organic air support capability, just like the Marines have. Of course, one reason the Navy-Marine Corps relationship is so successful is that the Navy knows their place: moving the USMC from point A to point B.