The Origins of the Acoustic Guitar
The birth of the acoustic guitar

Although steel-stringed acoustic guitars are now used all over the world, the person who is thought to have created the first of these guitars was a German immigrant to the United States named Christian Frederick Martin (1796-1867). Guitars at the time used so-called catgut strings created from the intestines of sheep. Martin, however, decided to create a guitar that used steel strings so that banjo players playing the then-popular style of country music could switch over and play without any discomfort. He is said to have created a brand new type of guitar by cleverly designing the inner structure to withstand the tension of the steel strings.
Reference: "The Fun Guitar Encyclopedia" (Yamaha Music Media)