The B-18 was designed shortly after the Douglas DC-2 airliner, but had little in common other than wing structure and tail elements of the DC-3. First-series B-18s featured a rounded nose, but the B-18As had the nose redesigned in a manner resembling the "Shark-Nose" 1938 Graham automobile. B-18As reached production in April 1938, though design was probably essentially completed in 1937.
The Graham (company history here.) was styled by Amos Northup but, due to his accidental death, some details were designed by other hands. The concept of the hood-grille ensemble was Northup's, however, and this work was probably done in 1936.
So even though the two designs were revealed late 1937 or early 1938, there is no reason to believe that one inspired the other. What we have here is a curious design coincidence.