In WWII-era films and newsreels, aerial bombs are often shown making a whistling sound as they fall; starting high pitched, then decreasing as the bomb approaches the Earth (example here, starting at about 0:50). I also remember it being mimicked in WWII-era Looney Tunes and the like that I saw as a kid (okay, and as an adult).
Did WWII-era bombs actually whistle like this? Why did they do so? Was it by design? I presume the bombs were falling slower than the speed of sound, so was the whistling audible from the ground - and soon enough to seek shelter?