*putting my WW2 history buff hat on*
Interesting stuff, guys. Esp. given that the Germans only had one operational bomber considered a long-range "heavy" bomber (though still just two engines), the Heinkel He 177:http://en.wikipedia.org/wiki/Heinkel_He_177
*That didn't enter service until 1942.
Now if you read German prototype and project plane lists, they had all sorts of heavy bomber plans that never made it to production.
In other words (as I understand it anyway), most of the Blitz were by bombers more equivalent to say, a medium U.S. B-25 Mitchell, rather than our B-17/B-24s or the British Lancasters of the 1940s. You can only imagine what sort of horror the Germans might've wreaked with, say, hordes of B-29-ish heavy bombers back in 1940 -- I venture something like that would've been tougher for British Spitfires/Hurricanes to down quickly -- but thankfully we didn't find out.