The sousaphone is a brass instrument designed for use in a marching band. It is essentially a marching tuba, and as such is usually in the key of BBb[ 1 ] though it may be found in C, F, or Eb. It was originally designed by the J.W. Pepper Company in 1893 at the suggestion of John Philip Sousa who thought that the helicons that he used in his band were less than ideal for concert purposes. He wanted an instrument that was portable like the helicon, but could be converted to have a sound more appropriate for concerts. Pepper named his invention the sousaphone in recognition of Sousa's suggestion. A common misconception is that the instrument was invented by the C.G. Conn company. The truth is that though Conn did provide the actual sousaphones used by Sousa's band the design itself was Pepper's. One other note is that the original design had an upward facing bell. The modern sousaphone has a forward facing bell, this modification was made in 1908.
source: http://www.jwpepper.com/history/sousa.html
[ 1 ] This has been a point of confusion for several people, let me explain. The term BBb is intended to differentiate between simple Bb which is the key of the euphonium and the key of the sousaphone which is an octave lower. The term double B-flat might also clarify matters.