The version that was popularized and sung by civvies during WW2 was copyrighted by Jimmie Hughes and Frank Lake in 1940, as has been noted before. Several parodies also have been copyrighted. See thread 10366 for comments and the original 1916 version by Fred Godfrey (not copyrighted, and not published because of the content). World War II

As far as I know, all of the military unit versions are free and clear. Here is the Coastal Command version:

Lyr. Add: BLESS 'EM ALL (Coastal Command)

There's many a Hudson just leaving Norway
Bound for old Iceland's shore,
Heavily laden with terrified men
Scared stiff and prone on the floor.
There's many a Heinkel a'pumping out lead,
And many a Messerschmidt too,
They shot off our panties,
And mucked up our scanties,
So cheer up, my lads, Bless 'Em All.

From "Airman's Song Book," ed. C. H. Ward-Jackson and Leighton Lucas, 1967 rev. ed., p. 136, Blackwood and Sons, Ltd..