OVERVIEW: The Mk 77 is an unguided, free-falling 500 lb incendiary bomb designed for area denial, anti-personnel, and anti-materiel use through intense fire and thermal destruction.

DETAIL: The Mk 77 series was developed to replace older napalm bombs (such as the Mk 47) with a more stable, less toxic incendiary composition. It uses a blend of hydrocarbon fuel thickened with a polymer additive, designed to cling to surfaces and burn at extremely high temperatures. The bomb casing is a cylindrical aluminum container fitted with a mechanical impact fuze and internal burst charge to disperse the incendiary mixture on impact. Mk 77s are stabilized with tail fins for aerodynamic flight and reliable ground impact.

The primary differences between Mod 1 through Mod 6 variants involve refinements in casing materials, incendiary composition (e.g., replacing traditional napalm with NP2 fuel gel), and improved safety during storage and handling. The Mk 77 Mod 5 and 6, for example, include reduced environmental persistence and are more compatible with current aircraft stores management systems. The overall explosive component is minimal; the damage comes primarily from sustained incendiary effect.

Mk 77s are typically used in close air support or tactical strikes by the U.S. Marine Corps and Navy. They are employed from low altitude to ensure accurate targeting of fortifications, vehicle concentrations, or infrastructure. Their deployment has been controversial due to the destructive and psychological impact of incendiary weapons.

NOTE:
IOC: ~1960s (Mod 1), with ongoing updates through 2000s
Operators: United States (primarily USMC and Navy)
Platform: AV-8B Harrier II, F/A-18 Hornet, A-6 Intruder, F-4 Phantom II, F-35B Lightning II
Conflicts: Vietnam War, Gulf War, Iraq War, Operation Iraqi Freedom

SOURCE:
U.S. Air Force Armament Museum ; Jane’s Weapons: Air-Launched ; Department of Defense, Historical Munition Fact Sheets ; Federation of American Scientists (FAS) ; U.S. Navy Weapon Systems Explosive Ordnance Disposal publication
https://www.fas.org ; https://www.afmuseum.com ; https://www.navy.mil ; https://www.janes.com