Additional records...
1) The corporate records of Northwest Airlines are archived at the Minnesota Historical Society
www2[dot]mnhs[dot]org[slash]library[slash]findaids[slash]00110.xml
The 1961 O'Hare crash is specifically noted in the Accident Files, which also references a section of general info on accidents between 1929 and 1974.
2) There was also a Coroner's Jury inquest held October 4, 1961, for which a transcript theoretically exists. The Cook County Medical Examiner's office is responsible for warehoused inquest archives, which are public documents. BUT: be forewarned there are no end of complaints about how difficult, time-consuming, and expensive it is to get those records. Numerous genealogy researchers have expressed frustration with obtaining even simple reports, or cited charges of $5 per page of sworn testimony from inquests.
One researcher's attempts to obtain a large inquest file are detailed in this article, which gives you an idea of what you might be facing:
www[dot]chicagonow[dot]com[slash]chicago-history-cop[slash]2016[slash]02[slash]cook-county-medical-examiners-office-is-a-joke-especially-on-valentines-day
I did find a few articles about the inquest that discussed some of the witness testimony - will PM a Dropbox link for you to access the clippings. Though you've probably already seen them...
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