where did the phrase "unsung hero" come from?

I?ve had a look at this and the New York Times first used the phrase
in article in 1905, and in 1921 with a headline: ?Unsung Hero to Be
Bearer of Unknown's Body?. Interestingly, the UK?s Times Newspaper did
not use it until 1956. So perhaps its origin is in the US. Otherwise,
I have no information as to its origin. Perhaps another researcher may
assist you.

answerfinder-ga

The phrase also finds use throughout the 19th century, and I doubt a
single source can be identified.  However, an awfully good candidate
for popularizing the term might be Paul Dunbar, an African-American
poet born in 1872, who eventually became the Librarian of Congress. 
He wrote a poem (undate, but late 1800's) called The Unsung Heroes,
which begins:

THE UNSUNG HEROES       

  
A SONG for the unsung heroes who rose in the country's need,
When the life of the land was threatened by the slaver's cruel greed,
For the men who came from the cornfield, who came from the
    plough and the flail,
Who rallied round when they heard the sound of the mighty
    man of the rail...



Wish I could provide a more definitive origin of the phrase, but for
now, this is the closest I can come.

paf

Pluralise it, and you'll find an example in the Aug 01, 1916 edition
of The Times (UK).

Citation:
"Such a story as that told by "Fp." of life at Danafa, Nigeria, gives
a good idea of the work that is being done for the Empire by all sorts
of unsung heroes in all sorts of strange places."

I wonder if the NY Times archive has earlier examples of the pluralised term?

I don?t think I had my thinking cap on earlier. Fancy not trying it pluralised.

New York Times: Appears first in an article in 1899.
One in 1912 for the Titantic: UNSUNG HEROES OF THE ENGINE ROOM; Chief
Bell's Men Kept Titanic's Lights Burning Until She Went Down.

Dear Mesmonde,
Whereever the phrase comes from, it is a reference to the opening line
of Virgil's Aenied:  "I sing of arms and the man ..."  That suggests
that there could be much earlier sources for the expression "unsung
hero":  Civil War, maybe.  The various   quotations from different
contexts (US and UK) suggest that the expression was not original with
any of them.  (Hope you folks agree.)

What's interesting about this "Times" story from 1916, is that even
then, fp (today, fp-ga) was providing quality information.