The first horns were monotone instruments. In 1753, a German musician called Hampel invented the means of applying movable slides (crooks) of various length that changed the key of the horn.
In 1760, it was discovered rather then invented that placing a hand over the bell of the French Horn lowered the tone called stopping. Devices for stopping were later invented.
In the 19th century, valves instead of crooks were used, giving birth to the modern French Horn and eventually the double French Horn. It is debatable if it is possible to trace the invention of the French Horn to one person. However, two inventors are named as the first to invent a valve for the horn. According to the Brass Society, "Heinrich Stoelzel (1777-1844), a member of the band of the Prince of Pless, invented a valve which he applied to the horn by July of 1814 (considered the first French Horn)" and "Friedrich Blühmel (fl. 1808-before 1845), a miner who played trumpet and horn in a band in Waldenburg, is also associated with the invention of the valve."
According to A Brief History of Horn Evolution, "Double French horns were invented by both Edmund Gumpert and Fritz Kruspe in the late 1800s. German Fritz Kruspe, who has been credited most often as being the inventor of the modern double French horn, combined the pitches of the horn in F with the horn in B Flat in 1900.