Graene Boak said:
"At this stage of the war, Japan was totally incapable of manufacturing an aircraft of the size and complexity if the Ju.390."
This site is worth reading:
http://www.j-aircraft.org/xplanes/
The Nakajima G10N1 Fugaku ( Mount Fuji ), in particular stands out.
There was a prototype which had not yet flown bombed in it's production factory three months before the end of the War with Japan.