I assume that he was in the U.S. Army? This would make a difference on the markings used on the plane, and if you know where he flew his missions that might help narrow down the unit he was in and possibly suggest specific markings or colorations.
The fact that your grandfather's aircraft had nose art at all suggests to me that he was flying this aircraft later than 1941 since the peacetime Army generally did not allow such personalization of aircraft. The U.S. did not officially enter the war until December, 1941, and it wasn't until units were actually engaged in combat operations that the practice of noseart was allowed to flourish.
_________________I like Thunderbolts!
(Link to MySpace tribute page)120th Fighter Wing, Montana Air National Guard