But El Cid - WTF does any of that below have to do with the fact that Japan couldn't/didn't build heavy bombers???!!!
1) Japan did build heavy bombers. Perhaps you mean "didn't deploy heavy bombers in our sense of the term operationally"???? Both the Army and the navy managed a 4 engine type - although in the event neither was put into series production. [The G8N1 would have been, but the tooling and plant were destroyed].
2) Japan did plan to build bigger heavy bombers. The "Japanese B-36" is virtually identical to the US plane, except it uses tactor rather than pusher propellers, and it is years ahead of the US design (by which it mean the design was suitable for production much sooner). By identical I mean range/payload and weight/power and configuration data are almost interchangable for long range missions. There was this big difference:
the Japanese plane could not carry 20 tons of bombs - they NEVER planned for massive conventional bombing even with a monster bomber.
This ship was intended either to duplicate the role of other Japanese bombers (attack ships at sea; attack critical air bases - like the raids on the B-29 bases which caused so much grief we kept it secret for decades).
They were not intended for raids like the ones we conducted on Japanese cities.
3) Japan actually bought production rights - and the early conversions - of a German 4 engine bomber (the FW-200 - "the scorge of the Atlantic") but was unlucky in timing and never got possession of either planes or tooling. It is a strong indication of interest in a role for 4 engine bombers - although these would be heavy by Japanese standards - and not intended for a US 8th Air Force type role.
4) Japan actually built "heavy bombers" in numbers and used them in terms of its own doctrine and terminology. We don't happen to understand that doctrine and terminology - so we think they were "wrong." But different is not the same as wrong. Japanese heavy bombers had three or four engines and, outside of China, were not really used in city bombing. Note I have MORE respect for Japanese bombers than for US ones - and my parents BOTH served with B-17s - I grew up in a "bomber" family. [USAAF WACs were first tranied as photographers - the very first job other than nursing for women in the US Army - ending up training gunners and bombradiers - they were first trained as combat intelligence photographers - in case the war went badly this was to free men for offensive missions.] The idea you use bombers efficiently against military targets does not offend me. The idea you use them to force civilians to demand peace - when in fact they make people who otherwise have no use for the regime to volunteer to man AA guns, aircraft spotting posts, or work in war factories. I don't believe in bombing civilians - not only because it is wrong - but because it doesn't work!