Even though the cars in this movie are all American, I think this bike is British - maybe a Norton?? It looks like a Harley Davidson to me, but I can't be sure about the model (perhaps a model B single, but it could also be a model D twin). Are there any better pictures that show the engine? http://www.youtube.com/watch?v=-y0uYfY7kOI
This scene was the reenactment of the attempted murder of Hale Keith; a off duty motorcycle policeman who was shot by Baby Face Nelson during the Sioux Falls robbery. In shooting, Hale Keith was hit in his; abdomen, right leg just six inches below the hip, his right wrist and arm just below the elbow. And like this scene; as Keith fell against his bike, Nelson did laughed before shouting "I got one" twice.