For my money, none of the B-17's, with the possible exception of the British B-17C's, were painted any shade of green.
I've seen pictures of the early 17's with a dark green, quilted sound proofing material, from the bombardier position to the radio room. I believe that this was deleted as a fire hazard.
The bomb bays and the waist positions don't ever appear, to me anyway, to have any all over coat of green. I have pictures of the waist positions of the major models, E, F, and G that shows them to be unpainted. You can read the stenciling denoting what type aluminum it is, in this case Alclad 24S-T.
There is a beautiful wartime color photo of a B-17G waist gun position with the framed, enclosed windows that the Monogram kit has. It shows the area to be unpainted.
And if you look closely at the waist position in Hans von Hammer's 'Little Miss Mischief' reply, you'll notice you can see the reflection of the ribs on the skin.
Is it true that all 17's were unpainted inside? ... No! But my reasearch indicates than most 17's were unpainted.
HTH
Tony H.