[edNorth]

Short answer may be because the type was also passenger version, in the beginning, for people look out and let light inside. After it was bomber, these were not strictly needed anymore. But one can see the ESAC bomb carriers inside the window on many pictures. So progreesively they were less glass, but were retained in production, I think they were opeanable (had hinges) and could be used as escape windows or went fuel fumes from leaking fuel cells.. Less glass as time went by (by H-5 these were metal). Longer answer has not been written yet. -Ed