The projects for the Do F (civil) and Do N (military) started around 1924, after the first commercial
successes of the Do Wal. Principally a Wal without sponsons, it nevertheless would have been new
developed, heavier aircraft, powered by either RR Condor III or Napier Lion V engines. Later, the
BMW VI was intended, too. Although development of the Do F seems to have started earlier, than
that of the N, the demand of a heavy twin engined military a/c by Japan probably lead to antedating
the military type, for which no construction work was necessary, as only raw materials and pre-products
were delivered to Kawasaki. In the meantime, the F had been developed further, again increasing
size and weight, now to a span of 28.50 m.
That's the detaily mentioned apart from those already posted.
The book is highly recommendable, but some knowledge of the German language is recommended,
as apart from the described types, the most interesting part, I think, are the relations of those manu-
facturers (Dornier, Gotha, Staaken, Rohrbach) to the Zeppelin group.
Will try to make a genealogy, I think ...