The German proximity fuse.

As far as I can see the only aircraft the allies would have that could
survive above German airspace and much of Europe would be the Mosquito.
The B29 would eventually come along but it wouldn't be available in
quantity till late 1944. Becuase of the high casualities the American
would have to cut their B17 based bombing rates in half and opperate at
higher altitudes. The lack of B24s means no Ploesti raid.

Presumably Wellingtons could be fitted with two stage supercharged
merlins and a presurised fueselage.

Basically I think you are right. Bombing altitude would increase from
the usuall 20,000-25,000 to 35,000-40,000 to avoid casualites. Medium
altitdue aircraft such as the B26 Marauder and B25 become outmoded.
German industry would reamin more productive and strong. Bomb gudience
becomes essential.