The Combat Infantryman Badge began life as the Combat Assault Badge on 07OCT43. The awards name was changed to its current title on 27OCT43 in War Department Circular 269.
On 08FEB52 the Army Chief of Staff approved the addition of stars to the badge to indicate involvement in multiple wars. Regulations now exist for the awarding of eight stars although I doubt any one is still alive with that many awards as we haven't had that many eligable conflicts in the last century. The first four awards are silver stars, with one added for every award. The fifth award changes the badge to gold as opposed to silver and has no stars. The sixth through the eighth award adds a gold star for every subsequent award. The CIB supercedes the Expert Infantryman Badge and will replace it on the uniform if the soldier qualifies for both.
The background is a light blue called "Infantry Blue" and is the same shade as the Infantry cords worn from the epaulet of the Class A dress uniforms of Infantry soldiers.
The silver (or gold) musket inlaid on the badge is the 1875 Springfield Arsenal musket adopted as the official insignia of the Infantry in 1924.
The CIB is awarded to "personnel in the grade of Colonel or below with an infantry MOS who have satisfactorily performed duty while assigned as a member of an infantry unit, brigade or smaller size during any period subsequent to 6 December 1941 when the unit was engaged in active ground combat." Although there is no mention of a minimum length of time served in combat, the Army is traditionally very picky about who receives this award. My unit was only deployed to Saudi Arabia for three months and we were told that had we stayed for an additional three months we would have received the CIB, but as such we did not qualify.
The Deputy Chief of Staff for Personnel changed the requirements for the CIB on 05APR63 such that "Any officer, warrant officer or enlisted man whose branch is other than infantry who, under appropriate orders, is assigned to advise a unit (South Vietnamese) will be eligible for this award provided all other requirements for such award have been met." This is, as far as I know, the only exclusion for non-Infantry personnel to receive the CIB and the wording would suggest that it only pertains to the Vietnam Conflict.
During WWII and for six months after any enlisted man awarded the CIB was entitled to an extra $10.00 a month of pay.