This is an early, and rare, D variant. Note the different tail shape and belly gunner structure, the lack of a tail gunner and how there are fewer guns overall.
This specific aircraft, the "Swoose," saw combat in the Pacific. It's the oldest surviving B-17 and the only surviving D variant. It's under restoration at the National Museum of the Air Force in Dayton, Ohio.
B-17s of the Eighth Air Force dropping their payloads over railroad yards in Donauworth, Germany in April, 1945. The smoke is a signal marker from the lead plane to signify it was time for the other aircraft to drop their bombs.
From the National Archives: "Lt. M. J. Bleakman, 0-724064, Baltimore, Maryland (right) makes final inspection of B-17 Flying Fortress before piloting Hampton Roads Port of Embarkation Historical Staff over the Port Area."
Many of the B-17s in museums are aircraft that never saw combat. The Castle Air Museum in Atwater, California has one that was used as a training aircraft. It's painted in remembrance of one shot down over the North Sea.
Not surprisingly, Boeing's Museum of Flight in Seattle has an example, a rare F variant. Built nearby, it served as a trainer aircraft and was stationed in Britain at the end of the war. It didn't see any combat, but was later used as a tanker and even appeared in movies such as 1970's Tora! Tora! Tora! and Memphis Belle.
The incredible Pima Air and Space Museum in Arizona, easily one of the best air museums in the world, has a number of famous bombers. The "I'll Be Around" sits in its own hangar, unlike most which sit outside in the brutal desert sun. This aircraft was first registered to the US Air Force, then served in the Navy, Coast Guard and with various private owners including as a fire-fighting air tanker.