On the aircraft. Like I said, they are different.
The Air Force has its origins in the Army Air Force. It didn't take long for the aviators to want to be their own service. Part of that campaign was arguing that the rest of the Army wasn't suited to run the aviation portion.
They separated themselves in every way they could. One of those ways was to use a different designation for aircraft than for other Army major end items. The alphabetical designations explained above pre-date the creation of the Air Force.
Once the seperation was accomplished, the new service was given the responsibility of procuring all aircraft - including the unarmed aircraft that the Army was permitted to have at the time. So, the designations stuck and are still used.
And, of course, aviators still consider themselves to be different.