Taken at face value I'd point at Dynasoar's post above. IMO it would have carved a niche for itself if the war hadn't interfered, but economics just weren't in it's favor. TBH competing with the DC-3 is just REALLY hard - it was just about the perfect aircraft for the era, capable of just about anything to some degree of success and produced in truly immense numbers as a direct result. Really though the thing that hurt the stratoliner more than anything was the war. It wasn't ever likely to sell in the thousands (although neither did the DC-3, civil versions produced as such were only around 600) but it was, and would have remained for a few years, easily the best option for transcontinental service in the US. At the same time, the DC-4 was on the horizon and while it didn't get factory pressurization until the DC-6 this was theoretically an option (a realistic one as well - Canadair did build their North Stars - otherwise just Merlin powered DC-4s - with it) my guess is that a lot of companies would hold out for the more familiar product with a decent degree of commonality.
The best shot would be a Stratoliner that drops the pressurization (or at least is much more a provision for it than a standard feature), has a lot more commonality with the B-17 and gains enough range to comfortably fly Pan Am's oceanic legs pre-war. Even then though it just sounds an awful lot like the DC-4E, which is fine except that the airlines would have thought much the same as they did about the 4E - too big, give us something cheaperl; at which point we're back to the production DC-4 and most companies inclination to stick with Douglas.