Music Newby--The second site Jon posted is interesting; the first one in my opinion is too shallow. A good standard source on any music history question is Grove's Encyclopedia of Music, a multi-volume goodie found in the reference section of many a decent library. From there and a few other sources, I have this to add:
The banjo came to America as early as 1675 in the minds of those Africans fortunate enough to have survived the "Atlantic Passage." When able, they built instruments which they had played at home, which may or may not have spread from the Portuguese settlements, Angola and Mozambique. These banjos were five-string, fretless, and gut-stringed and had bodies made from gourds.
About 1830 a planter, Thomas Dartmouth "Daddy" Rice observed some of his stable hands doing a song-and-dance thing called "Jump Jim Crow" (Fireside Book of Amercan Ballads, etc.). Rice decided that had more entertainment value than just something for the slaves to amuse themselves with, and out of this, the white "blackfaced minstrel" troupes were born.
Having more resources, the whites decided they could improve the instrument. The gourd was replaced by a wooden hoop to which a hide was tacked, using carpet tacks with large, ornamental heads. Hence the name "tackhead" for this generation.
The tackhead lasted for only about a decade; it was replaced by the modern system of screw-down clamps holding the rim which in turn holds the skin to the head. Smithsonian has an example of this construction from no later than 1846.
Next major change was the steel string. This was a serendipitous off-shoot of the first efficient wire-pulling machine, not invented until the 1880's. The purpose was to make cheap steel barbed wire to sell to sodbusters, to keep the longhorns out of their turnips. Some bright lad suggested setting the diameter smaller and making cheap, strong steel strings to replace the gut and the occasional brass strings, which had been cold-hammered. (I cannot imagine sitting up late at night hammering a string into shape, but there you go.)
The wire-puller "drew" heat-softened steel through the adjustable aperature between industrial diamonds, hence the brand name (no laughter, please) BLACK DIAMOND for one early metal string manufacturer.
The steel strings seemed to beg the question of frets, which were added, or at least more commonly added, after the steel string. The fretted steel string changed the style, because it was no longer necessary to "knock down" on the whole battery of strings in order to be heard. Syncopated arpeggios etc. became more feasible, and banjo players changed from two-finger/clawhammer to three-finger styles, inventing all the various "rolls."
The only thing missing from that point was the clamshell resonator, which was added for the same reason the Dobro was invented--more volume.
I think anybody who said he "invented" the banjo is blowing smoke, like Jelly Roll Morton saying he "invented" the blues. Synthesized, maybe; invented, maybe. There is also a "White Supremacist" version of the history of the banjo, which credits an unnamed Confederate general with ORDERING the addition of the fifth string. In fact, the 4-string tenor became popular only in the Ragtime era.
As I say, go to Grove for the Pure Gospel, but the above is pretty solid, as my group does "trad" stuff and we have been over this ground a lotta times. (Actually a time, two times and half a time, but that's from another thread.)
Oh, I left out picks, aka "thimbles." I can't ever remember who gets credit; 'bout 1840-50, I think. Don't quote me on that part.
Chicken Charlie