Well, I'm struggling with this one..., but I'll post what I can..
1a. Early aircraft by Douglas included the DT-1 & DWC, along with the C-1.
1b. In order, Nov. 1921, Nov. 1923, May 1925.
2a. I'm assuming you mean the Douglas Cloudster..
2b. The Cloudster first flew on February 24, 1921.
3a. Donald Douglas's original company was Davis-Douglas Company, founded in 1920 and dissolved the following year.
3b. Bit of confusion for this second part.. "date of its maiden flight aircraft"?
What stands out to me with Douglas in those early years is how quickly the company changed: First producing a one-off flyer to obtaining a Navy contract to starting a new company to producing and flying that same year. Nothing in aviation moves that fast anymore.