The Donald R. Hammerstrom that died in the mid-air collision over Scotland was my grandfather's youngest brother. If this is the same Donald, he was married to a Cleora (not sure of maiden name) and she was pregnant with their son Donald Kenneth Hammerstrom when he died. D.K. Hammerstrom died in 1958. Both are buried in Fort Snelling Cemetery, in Minneapolis, MN. I was told she remarried some time later, but no further information.
I would love to hear any more information you have.