I did find this page for a crew shot down over Nantes, on a mission to bomb the submarine repair facility there.
Higdon Crew
That page shows the name of their aircraft as "Doris Mae." "Doris Mae" was 42-3218, assigned to the group on 29 May. If that was his first aircraft, it was given to the Armstrong crew when it arrived at the base.
Ken Cunningham is second from the right in the front row.
Kenneth Cunningham
He was assigned to the 546th Bomb Squadron (H) on 4 July 1943.
The Higdon crew was in the B-17F 42-3459 "Jolly Roger" when they went down, but that plane wasn't operational until 9 August 43, so it wasn't their original plane.
The Higdon crew flew their first missions in the following aircraft.
24 July 43: 42-30033 "Little America" - A/C assigned to group on 24 May.
24 July 43: 42-29927 "Wabbit Twacks" - A/C assigned to group on 6 July.
28 July 43: 42-29728 "El Rauncho" - A/C assigned to group on 12 July.
12 August 43: 42-29927 "Wabbit Twacks"
15 August 43: Unassigned spare aircraft - Replaced aircraft 42-29703 (No Name Recorded) - A/C assigned to group on 10 July.
It appears "Wabbit Twacks" was the most often assigned of the early aircraft. That was likely his first operational assigned aircraft, as it looks to have been assigned to the group only two days after he was assigned. The difference in dates would likely be from the aircraft and aircrew paperwork having to go through two different channels.
Here are a couple images from his sortie records.
Purple Heart and Air Medal
Wound Report
I hope this is your father and his aircraft. If not, I'll keep digging.
-Irish