The only 1/48th B-17s on the market RIGHT NOW that I'm aware of are the Revell (ex-Monogram) B-17G and Visible B-17G (same kit as the old Monogram, overall, but has a clear fuselage half and extra detail parts and figures to be viewed inside the bomber's fuselage.). Revell's releasing (or re-releasing) a 1/48th B-17F soon, but it's down the road a bit longer.
On Ebay you might find the 1/48th Pro Modeler B-17G (Monogram with PE, extra figures in flight gear, and a choice between the "stinger" tailguns and the Cheyenne tail) and the old release of the 1/48th Memphis Belle from Revell...
Everyone here doing 1/48th is doing one or other of the Revell/Monogram Forts... I think anyway... B-17 Guy can be confusing with his kitbashes/conversions... I've just started a pair of B-17Gs, one's the Revell/Monogram, one's the Pro Modeler...
Even though it's about 30 years old, the Revell (read: Monogram) B-17G is still a great kit and is still about as state-of-the-art as one will get in the foreseeable future... It's running close to 30.00 these days, but that's about as good a price as one will get unless you got a Hobby Lobby and their 40%-off coupon.