I have no idea what the other bomber flights were flying at, so maybe one of them was that high. As for historical accuracy in World War II B17s were preferred by many bomber crews over B24s because they had a much higher ceiling. Meaning they spent less time in range of the high alt ack guns.
I do know that 8th Air Force operated many of its missions in the high 20Ks .. not sure off the top of my head about in the low 30Ks.
As for Green flight (Nightmares and MAW), the MAW P47D11s were at 26K, the Nightmares P47D11s were at 23K, and the Nightmares B17Gs were at 18K.
So must be a different bomber group you ran into.