Stargazer,
I try here to explain a history fact,that's the Douglas company used two series
500 and 1000,but the 500 one was related to 100-400 series and followed it,
I can answer on your question;
- for 1000 series,it was begun at late of 1945,and not related to 100-999 series,and it
was called "Model" not "D".
- for D-499 (X-3 Stilletto) the request began at December 30,1943,and the design done
in June 30,1945.
- for D-509 (DC-4) was post war civil model of 1945,and not in 1938.
- for D-558 was in late 1945.
- for D-490 (XC-114A) was appeared in early 1945.
