It was designed to replace the M16 smoke grenade, which did not burn as long or as vividly. It was designated standard issue in the fall of 1943. Both were produced at the same time as the M16 production lines were already setup when the M18 was adopted.
It was designed to supplant the M16 smoke grenade, which did not burn as long or as intensely. It was designated standard issue in the fall of 1943. Both were manufactured concurrently with the M16 production lines already established when the M18 was adopted.