As Don Herzog points out, originalist views about judicial interpretation seem to suggest that it is:
Article I, sec. 8 of the Constitution enumerates Congress's powers; clauses 11-16 lay out war powers. We learn that Congress can establish an army and a navy. But there's nary a syllable "in the Constitution" about the air force!
With the possible exception of Ben Franklin, who had some amusing ideas about balloon warfare, the Founding Fathers never intended there to be an Air Force. Now if you're going to read the bit in Clause 1 -- "provide for the common defense and general welfare" -- broadly enough to allow Congress to set up an Air Force if it's important for common defense, you'll have to let Congress set up other national institutions if they're important for the general welfare.
