Seeing how your pictures were not done right here.

One of them was far to large....
(Being a photographer, I know that if that was another aircraft, the dark area would appear more as a sharp edge.... Even with 1940's film technology... Therefore I safely would bet those 17's are low)
You would lose that bet.....look at the photo you tried to post. Notice the trail dissipates in far less than a second? It takes extremely cold moist air for persistent contrails regardless of source.
Now look at the trails above the two B-17s, long with low definition, clearly indicating a persistent type of contrail. Also the fact that they are flying in formation would indicate that its very likely they were at high alt. In all likelihood this picture was taken from the top turret of a lower B-17 well above 20,000 feet.
Also note that the contrails are
not from the exhuast but from the propeller tips.
Strip