Apparently painted on a brick or concrete block wall. Possibly the inspiration for the lettering on this aircraft. However, to date there is no photographic evidence that this lovely lady appears on the aircraft itself.
An exact copy of this photo was on the original site (old pages). Description and contribution noted there:
B-17G 42-107148 "Dark Angel"
Shot down on the March 17, 1945 mission to Erfurt, Germany, piloted by W. Shauer.
Photo courtesy of Leon Briones.