GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Actually, Ju390V1 (GH+UK) was built from a Ju90 prototype airframe.The Ju 290 prototype flew on July 16, 1942. The Ju 290 entered service in August 1942.
The Ju 390 was a development of the Ju 290 with 6 engines and extended wings.
Ju390V2 (RC+DA) was built from a Ju290 airframe and components.
What I find interesting, is that the Ju390V2 was being built for long range maritime service and was said to have been equipped with a FuG200 radar system, yet in all the "airborne" photos of V2, no antenna are present.