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Of the two brothers, it was Joseph who first contemplated building machines as early as 1777 when he observed laundry drying over a fire incidentally form pockets that billowed upwards. Joseph made his first definitive experiments in November 1782 while living in the city of Avignon. He reported some years later that he was watching a fire one evening while contemplating one of the great military issues of the day—an assault on the fortress of Gibraltar, which had proved impregnable from both sea and land. Joseph mused on the possibility of an air assault using troops lifted by the same force that was lifting the embers from the fire. He believed that contained within the smoke was a special gas, which he called Montgolfier Gas, with a special property he called levity.

As a result of these musings, Joseph set about building a box-like chamber 1×1×1.3 m (3 ft by 3 ft (0.91 m) by 4 ft) out of very thin wood, and covering the sides and top with lightweight taffeta cloth. He crumpled and lit some paper under the bottom of the box. The contraption quickly lifted off its stand and collided with the ceiling. Joseph then recruited his brother to balloon building by writing, "Get in a supply of taffeta and of cordage, quickly, and you will see one of the most astonishing sights in the world." The two brothers then set about building a similar device, scaled up by three (so 27 times greater in volume). The lifting force was so great that they lost control of their craft on its very first test flight on 14 December 1782. The device floated nearly two kilometers (about 1.2 mi). It was destroyed after landing by the "indiscretion" of passersby.