Here is newsreel footage of the Douglas XB-19 (USAAF Ser. No. 38-471) under construction at Santa Monica. Donald Douglas and (then) U.S. Undersecretary of War Robert P. Patterson appear in the footage at the 26 sec mark, which would date the footage to sometime between December 16, 1940 (when Patterson was appointed to the position) and May 1941 (when construction of the airframe was initially completed.)
YouTube - British Movietone: "Giant Bombing Plane - No Sound"
And here is a real gem. This is 16mm color film footage posted by YouTube poster George Mihal (and what he's entitled the "Office of Image Archaeology") of the XB-19. The film (presumably taken/arranged by cinematographer Paul L. Hoefler) is of the aircraft being prepared for its first flight from Clover Field, Santa Monica, to March Field, on June 27, 1941, and includes footage taken from inside the aircraft in flight. Test pilot Maj. Stanley M. Umstead (USAAF) can be seen climbing into the aircraft at the 8min mark and poking his head outside the cockpit window soon thereafter.
YouTube - George Mihal: "27 June 1941, Douglas XB-19A, Howard Hughes, Paul Hoefler, Hi-Def"
As for the title of the footage, I have my doubts that Howard Hughes is the gentleman shaking hands with Hoefler at the 4min 34sec mark (that is the subject of that very short excerpt posted to YouTube by Mr. Mihal -
LINK), but that is for others to ascertain.