I'm a little late, but in 2010 Cory Bernart, in collaboration with the National Agricultural Library, created an exhibit entitled "When Beans Were Bullets: War-Era Food Posters" of posters from the WWI, the inter-war period, and WWII.
I've included a small sampling on this page and after the jump. You can see many more examples, including some videos, from the exhibit at the website: War-era Food Posters.