Hi, Bish:
I've been building B-17's since 1982, and I have 24 of them (plus 2 B-24's and assorted 8th AAF fighters--all1/72) stored in plastic boxes to keep them out of the dust. It was the year I discovered Hasegawa.
Chin turrets began appearing on some late-model B-17F's, and perhaps the only way to tell the difference between an "F' and a "G" for sure is by serial number. There were some "escort" B-17's, called the "YB-40," loaded out with guns which I think had chin turrets too. If I remember right, YB-40's were converted "F's."
Also, the side gun windows in the nose were often located differently, depending on whether the plane was built by Boeing, Vega or Douglas. It took a while to standarize the location. Then there was the factor of the modification centers (like Cheyenne, Wyoming), where some planes were altered before shipping overseas; plus any field modifications that were carried out once the plane arrived in England. It makes for a confusing mess, so I try to find a photograph of the plane--which isn't always possible and which doesn't always answer all questions. Most photographers seemed to be more interested in the nose art (I wonder why?), rather than whether the waist guns were staggered or not; and whether the guns mounts were K-5 or K-6.
The recoil from the chin guns tended to crack the longer "F" nose, so the cones were shortened during the "G" production runs, although there were "G's" that continued to carry the longer nose cone. In addition, some early "G's" made it into combat with the old window arrangement from the "E" series, notably and unfortunately "Pistol Packin' Mama" from the 91st Bomb Group, which means I can't build that ship without substantial modification. Maybe the best answer would be to get an "E" and splice in the chin turret, rather than try to fix the windows on a "G" kit. But not for this group build!
I have been partial to the Flying Fortress since the days Robert Lansing portrayed Brig. Gen. Frank Savage in "12 O'Clock High" on TV--waaaaay back in 1964/65 (when I was 12).