High enough to escape blast damage from their own drops, but very low, yes.
30 feet wouldn't be enough to clear many target buildings, although en route, im sure they went that low at times.
The one flying over the smoke stack is @ 200 feet up. Certainly low compared to the 15-20k they normally did.
Forgot to add: Its been done several times in AH with B-17s, dunno if they ever re ran it with B-24s. Was fun.