Although colloquially referred to as a napalm bomb, the ZB-500 is actually filled with a synthetic substance called Ognesmes which is a mixture of toluene, kerosene, and polystyrene. The ZB-500 weighs about ½ ton, and the ZB-250 about 600lbs. Besides the obvious flame and heat effects, burning Ognesmes consumes a huge quantity of oxygen out of the air presenting a secondary danger to personnel in the area.
Both weapons are basically shaped like drop tanks with no fins, save a small stub which serves to prevent the bomb from oscillating against the belly of the drop plane during flight. They are intended to tumble end-over-end when dropped, allowing the plane additional time to escape if used at low altitudes. Both bombs have very thin skins. The Ognesmes is ignited by a small white phosphorus pack attached to the fuse. There is no bursting charge, with the bombs being dependant on their own velocity to shatter open and disperse the burning payload.
There is anecdotal evidence that the USSR used these bombs during the 1980s Afghanistan war but this is not confirmed.
It should be noted that the Warsaw Pact classified the ZB-___ bombs as “chemical weapons”; in Russian this term does not carry the same moral connotation as in English and the Soviet Union classified not only poison gas, but also napalm, smoke screens, and flares in this category.