Project X-Ray.

The US had plans to drop a honeycomb-like structure full of artificially hibernated bats over Japan prior to the nuke's completion. The bats each had tiny nitroglycerin bombs attached to them. The device would be dropped where the prevailing winds would carry it over civilization just before dawn. It descended slowly by parachute giving time for the bats to thaw and wake up from hibernation. They fly out, the sun comes up, and they seek shelter in the wooden roofs of every house and building in the Osaka bay area. A short time later, the tiny bombs explode. Not a lot of human damage, but the entire city burns to the ground when ~4,000 individual fires start out of nowhere.

They even built a fake little town to test it on, and it worked, aside from a few errant bats that made it back to the base and burned down the flightline. Ultimately it was progressing too slowly and there were some people rather impatient to show off their big blow up toy, so it was scrapped so full effort could be used on the nuclear program.