Mesh has holes I can't get rid of (already tried re calc normals)

I’m am modelling boeing 747, which is about 85% complete but there are 2 holes in the fuselage which I cant see in blender but when I export to unity they become quite noticeable. I have tried normals > recalculate outside & I have tried to select the 2 faces > Normals > flip direction, none of which works.
I created the model using a mirror modifier which has been applied with clipping on, but even before I applied the mirror modifier It still had the holes in it. Something must be quite messed up but not sure how it came about.
Anyway any help / suggestions would be very much appreciated.

Model as seen in unity :

Blend file :

747_forum.blend (350 KB)

Your model has big faces with more than 4 edges on the underside below the wings. Get rid of those and replace with quads and tris.
I assume Unity renders objects as triangles so you’re relying on some software to convert your ngons to triangles. You have a brain so have a better ability to do this.

Well after doing most of what JA12 suggested and removing some Ngons (some still remain) I no longer have any holes in my mesh, it looks much better in unity now although I’ve not done anything with the nose yet, It’s far from perfect I know but overall I’m happy with how it’s looking.

Thanks a lot guys, this was driving me crazy.