Why did the design of the Boeing 247's cockpit windows change for some models?
From the Boeing 247 article (emphasis mine):

The cockpit windshield of the first 247s was angled forward, instead of the conventional aft sweep. This was the design solution (similar to that adopted by other contemporary aircraft that used a forward-raked windscreen) to the problem of lighted control panel instruments reflecting off the windshield at night, but it turned out that the forward-sloping windshield would reflect ground lights instead, especially during landings and it also increased drag slightly.[13][14] By the introduction of the 247D, the windshield was sloped aft in the usual way, and the night-glare problem was resolved by installing an extension (the glarescreen) over the control panel.[15]