You found a subject of great interest and at the same quite unknown. However this matter is covered in utmost detail in "Der Japanfug" by Peter Herde, 2000. It is a 303 pages book, covering in detail the various plans to establish air contact between the Axis powers in Europe and their ally, Japan. The text is in German. I must admit, it is a MOST interesting book.
There have been plans, projects and plane prototypes build for this purpose by Germany, Italy and Japan.
These concluded with some actual flights.
- Rome - Tokio - Rome, leaving Rome June 29, 1942 and returning July 20, 1942. "El Duce" Mussolini was at arrival airport "Guidonia".
- Tokyo - Singapure - Sarabus - Berlin - Tokyo, leaving Tokyo on June 7, 1943, but disappeared over the Bengal Golf. No enemy action involded.
- No German Flight was made, because by the time a machine was ready, the situation of the Russian front had changed and certai airports were no longer usable.
Hope this helps for your further studies. Take care. Walter Godinho.