This is a model of Volkshalle, the People's Dome, part of a vision of "Germania" by Adolf Hitler. Unfortunately, the tides of war had washed Germania away, but this and other architectural plans by Albert Speer were influences of many alternate history fictions, most notably The Man in the High Castle.
If off the drawing board, the dome itself would have been 850 feet wide and its interior would fit roughly a million people. The one detail that I always get puzzled on is that it would be so large that the exhalations of so many people would turn the interior of the building literally into a raincloud. Where did this assumption come from? And if true, could there be some way to counter this meteorological complication?