The idea for the balloon bomb had been floated around for decades. It was first tested in the Italian War of Independence in 1848. A group of Austrian soldiers attached small bombs to primitive hot air balloons. The balloons floated around for 30 minutes before they detonated. They weren’t very effective and couldn’t be controlled very well. Sometimes, the balloons even flew back into the Austrian troops!
This picture is an actual hand drawn diagram of the Austrian balloon plans: