Available on tubi
You can watch this on the free streaming service tubi with some ads right here:
https://tubitv.com/movies/636895/batman
Unlike the 1949 serial, which is also on tubi, this isn't broken up into chapters, it's all there so you don't see a building blow up and wonder if Batman bit it because you see him get away all in the same scene but still, it's a chance to see it how Batman was imagined on film at the height of WWII.