Quentin Reynolds, "A London Diary" (1941):

"The [RAF] boys who fly the Lockheed Hudson bombers and dive bombers over Norway [in 1940] have a grand song almost one fifth of which can be printed. It is sung to the tune of an old English classic, ‘There’s a troop ship just leaving Bombay….’ It goes (allowing for changes):

        They say there’s a Hudson just leaving Norway
        Bound for old Scotland’s shore
        Heavily laden with terrified airmen
        Bound for the land they adore.
        There’s many a Heinkel around in the Skagerrak
       And many a Messerschmitt too
        You’ll get no promotion that side of the ocean,
        So cheer up, my lad, bless ‘em all.

        Bless ‘em all, bless ‘em all—the long and the short and the tall.
        Bless all the sergeants and W.O. 1s—their blessed daughters and their blessed sons - etc.

"Low and I can’t sing as well as Beattie, but we can sing louder."

Reynold's dates this journal passage "Oct. 1," which would be weeks earlier than the earliest newspaper mention of the pop song (Dec. 12, 1940) that I've found.

FWIW.

(Google Bard insisted that "BEM" was released on March 15, 1940, citing several websites, but when challenged confessed it had no idea of the precise date.)