The first significantly operationally used cluster bomb was the German SD-2 or Sprengbombe Dickwandig 2 kg, commonly referred to as the Butterfly Bomb. It was used in World War II to attack both civilian and military targets. The technology was developed independently by the United States, Russia and Italy (see Thermos bomb). The US used the 20-lb M41 fragmentation bomb wired together in clusters of 6 or 25 with highly sensitive or proximity fuzes.

https://en.wikipedia.org/wiki/Cluster_munition