“He was all the way in the back, with those twins guns. ...”
No he wasn’t.
The Doolittle Raiders flew B-25Bs, which were painted olive drab, had a red dot in the center of the white star on the national insignia, had no square windows for waist guns, had their dorsal turret farther back (about even with the rear tips of the nacelles - would be visible in this rear quartering view from below), and did not have cheek guns.
Not sure which version this is, but the first to mount cheek guns was the B-25H.
From the article: Corporal Thatcher, a 20-year-old from Montana, manned a pair of .50-caliber guns in the raid... He rode in the rear of the Mitchell B-25 medium bomber...
Maybe you should let the family know great-grampa was a liar.
And another view of a B-25B, this one flown in North Africa:
http://militaryhistory.about.com/od/worldwariiaircraft/p/b-25-mitchell.htm