Whence does the 1939 Howard Hawks film, "Only Angels Have Wings" draw its
title?  I believe the title was originally printed all lower case, as
though it were at the end of a sentence, implying that it was a
reference to something.  My question: what was that something?

I cannot prove it, but I've always taken this as a reference to an old
saying, "Both men and angels fly toward God, but only angels have
wings." This saying does not appear in the screenplay, however.