I think the Pentagon might have held up against the airliner if they had not put windows in the Exterior walls.
My thoughts on the matter for what their worth.
All of those holes in the wall are stress locuses.
The windows are actually blast resistant, but the plane even breached the concrete structure itself making a hole 75 feet wide. The outside wall collapsed some time after impact but the initial size of the hole could determined by the number of interior support columns the plane destroyed.