There is of course no doubt the plane hit the Pentagon. There are many witnesses, including three of their star north-of-Citgo witnesses, Lagasse, Turcios and Brooks, who all said they saw the plane hit the Pentagon. Sean Boger from the Control tower at the Pentagon Heliport watched it all the way into the Pentagon. Albert Hemphill at the Navy Annex said it was to his right, and flew straight over the bridge into the Pentagon. There is not one witness to the plane flying over the Pentagon though there were hundreds of cars stuck in traffic around the area.
There is radar data from four installations all leading directly to the Pentagon impact site. Now there is also the FDR file, which has recently been fully decoded, showing the plane flying straight, at the same track angle as the radar, and descending. The last radio height is 4 feet. Add 6 feet for the offset and 16 feet for the height of the wings above the wheels and you get a height that matches the length of the chopped off light poles.
So all the evidence shows the plane hit the Pentagon. Its mass is sufficient to smash the wall. Most of it went inside. There is enough debris outside to account for the parts that were too light to penetrate. The claim the plane flew north-of-citgo is a claim based on no evidence other than a few who were mistaken about something that would not seem important to remember at the time.
You can read about this here:
The Science of 9/11| Pentagon