The specific clauses in question are in Article 1, Section 8 of the Constitution:
Note the bolded sections. Army appropriations can be for no longer than two years. Also note the extensive powers concerning Congressional powers to support militias. Taking these together it's clear that a standing Army, as we have now, was not what the Framers intended our military to be.
Meanwhile note the Navy clause. It's literally a single line, with no limitations ascribed to it: "To provide and maintain a Navy" (which, note, was understood to included the Marine Corps even back then).
It's pretty clear that the the Framers expected the US Government to have a standing Navy, and even back then proper warships took years to make and were maintained over decades and they knew it, so they structured the Constitution to reflect that.
Now, I'll grant, time and warfare have changed. The US Army uses vehicles that require longer term financing and maintenance similar to how the Navy needed in the 18th century. So really the Constitution probably should be amended to allow for longer term procurement for the rest of the military, but as it stands now... yeah... the Army and Air Force are both on Constitutional shaky footing.