The Flight Engineer/top turret gunner is the senior NCO and responsible for a.) the load out on takeoff, b.) positioning the crew at the radio bulkhead station - then he moves forward across the cat walk to his station behind pilot/co-pilot for takeoff. Comer is first guy that should be chewed out - the ammo was probably stored back there on Comer's orders as it is hard to image that the aft crew moved ammo around and then watch the tail gunner maneuver past the stored ammo at bulkhead as the B-17 was taxiing.
It is a strange tale.
Note - we may both be a little dyslexic regarding the names and who told who. I assumed (bad) that 'Jim' was the Flight Engineer, and that the story teller was the co-pilot. If the story teller is Comer, and comer is top turret gunner, then who is Jim of 'Jim should know better'? Once again the top turret gunner/flt engineer is senior NCO
EDIT - OK - I have it now. Kels is co pilot, Comer is Flt engineer - so 'Jim' must be some other NCO still aft of bomb bay..I still suspect Comer ordered the ammo that wouldn't fit on radio compartment/bomb bay bulkhead to be distributed toward the waist. There is no explaining "Legg" being stupid enough to crawl to his position - not a great place to be if the B-17 crash lands on takeoff... if airplane survives then he spends more time crawling out over tail wheel than simply egressing out the aft door like the rest of the aft crew.
Gleichauf Crew B-17G-10-DL 42-37719 "Hellcat":
John Comer, flight engineer/top gunner and senior NCO wrote the book, and no he did not "order" the crew to redistribute ammunition boxes into the waist; that is yet another assumption made with absolutely no evidence whatsoever.
"Jim" is Sgt James Counce, senior waist gunner who could also double as a flight engineer. The other crew members in the waist/tail area were Sgts Wilson, waist; Harkness, ball; Legg, tail gunner.