Mike,
You'd be looking for the Individual Deceased Personnel File of 37307545 S/Sgt Nies. These used to be held at Maxwell AFB. You could apply under the Freedom of Information Act (no, I'm not American either) where it will take 12-14 months. They have now moved shop to Ft Knox KY. I have a couple of them. On average 200 plus pages. I was looking at an American rear gunner, who was the sole non RAF man aboard, so besides the American stuff I was provided with the MRES documents as well.
NARA: You'll need to access the Graves Registration Units of the US Quartermaster Corps. I don't recall the RG number off the top of my head.
Finally, have a look at the Missing Air Crew Report for his aircraft, B-17F # 42-5411 https://catalog.archives.gov/id/90914658
Regards,
Dave
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