You have to search for Boeing or Flying Fortress at Fiddlers for it to show. It's 19.95 and is a type G as previously stated.
As for the differences. If I recall correctly the B-17F was the first to use a full plexiglass non-ribbed nose cone (in later models) and off-set cheek guns, larger bomb bay (around 8000 lbs I think) and extra fuel capacity (not that you'd notice that in a model) I think later F variants included the remote chin turret after experimenting with various extra guns in the nose; which became the defining silhouette for the G variant with the G being the last model made I believe.
Looking at the design differences, I don't think it would be easy - without major surgery - to remove the chin turret from a B-17G and re-model the nose to fit in the the Memphis Belle B-17F which has no chin turret and a framed glass nose rather than the single piece moulded version of later models. Not that it couldn't be done, but it would be a fair challenge. It looks (from a couple photos I have) that the Memphis Belle had four cheek guns, two mounted in the nose frame and two place back in offset line at the cheeks.
Happy to be corrected if I've got any of that wrong.