A Dobro is actually a brand of resonator guitar. Resonator guitars were invented in the United States by the Doyera brothers, who founded Dobro Manufacturing in 1928. It was the result of trying to build a louder guitar in the pre-amplification days. They are easy to spot as they are acoustic guitars with a large metal resonator cone built into the body. They sound closer to a banjo than an acoustic guitar because of the metal resonator.
Resonator guitars were also made by the National String Instrument Corporation in the 1920s and 1930s. There was a U.S. patent conflict between the two guitars so Dobro Manufacturing changed their design. and set the standard for single resonator guitars. In later years, other companies also built the instruments. As time passed, the brand name "Dobro" passed into common usage to describe all resonator guitars, no matter who manufactured it. In 1994, Gibson Guitar bought the "Dobro" trademark iand announced they would defend the right to the exclusive use of "Dobro".
According to Lee Dickson, Eric Clapton’s former guitar technician, Eric plays an old resonator guitar that was purchased many years ago. It is a composite of two separate resonator guitars. The neck is a Dobro (the brand of resonator guitars) which Eric had fitted to a resonator guitar body from an unknown manufacturer.