Copyright can be a very gray issue, and in the final analysis, it doesn't matter what copyright law says, it only matters what a judge decides in a copyright lawsuit.
Having said that, the photos from the U.S. Army Signal Corps collection in the U.S. National Archives (Record Group 111-SC) are considered to be in the public domain. You can Google National Archives, and probably find an explanation of this, along with some of the Signal Corps photos that have been digitized.
Additionally, the Bundesarchiv in Germany donated a large photo collection to Wikipedia, and they consider these photos to be in the public domain. (Read here http://en.wikipedia.org/wiki/German_Federal_Archives
, and the links for Wikimedia Commons, particularly this one on reuse - http://commons.wikimedia.org/wiki/Commo ... _Wikimedia
While looking this up in Wikimedia, I saw that the National Archives is doing the same - see http://commons.wikimedia.org/wiki/Commo ... Categorize
and http://commons.wikimedia.org/wiki/Commo ... 754_-_1954
- some of the WW2 pics are in here, but not many (so far).
Best of luck with your project!