While not a war poster, I feel this entry is within the spirit of this thread.
In 1942, the commanding officer of Coast Guard Cutter 83359, an anti-submarine patrol "sub-chaser" based out of Staten Island, New York, wrote to Walt Disney directly, asking Disney to draw a emblem or insignia for his vessel. He had heard that Disney was creating all kinds of illustrations for many different units in the armed forces. He suggested that either "Pluto the Pup, Dumbo, [or] Oswald the Rabbit" be used, "clutching in one paw (hand) a can of T.N.T." and riding a sea horse. Disney was always happy to oblige. Above is his creation for CG-83359 and her crew.
Here's a poster of an 83' patrol boat. hunting a sub.