W41 (nuclear weapon)
- B41Y1: "dirty" high-fallout with a yield of 25 MT
- B41Y2: "clean" less than 10 MT yield.
The B41Y1 was the only three-stage device ever built by the U.S. As opposed to the usual fission Primary and fusion Secondary of a Teller-Ulam design, this bomb's uranium tamper constituted a Tertiary.  The Y2 version lacked the Tertiary.
In the Y2 version, the device had the highest weight-to-yield ratio of any U.S. nuclear weapon. Carey Sublette quoted Theodore Taylor as saying the maximum practical ratio was 6.0KT yield per pound of nuclear weapon; the deployed weapon, with parachutes and other weight that was not part of the ratio calculation, was 10,670 lb. Test devices from which the B-41 was weaponized, however, were in the 9,100 pound range, consistent with Taylor's estimate.
Approximately 500 were built, between September 1960 and June 1962, and were in service from November 1963 to July 1976. It was replaced by the 9MT B53, which also was implemented as the W53 warhead for the UGM-27 Titan II intercontinental ballistic missile. 
- Globalsecurity.org, B-41
- Sublette, Carey, "The B-41 (Mk-41) Bomb: High yield strategic thermonuclear bomb", Nuclear Weapon Archive