The Barclay's 20+ Year Treasury Bond inception date was July 21, 2002. You aren't going to find treasury bond information going back to 1900 because Treasury Bills have only been issued since 1929. The U.S. Department of the Treasury will give you data back to 1990.
There's a good article in the Globe and Mail which covers why you may want to buy bonds as part of your portfolio. The key is diversification. Historically, stocks have done better than bonds long-term, but when stocks fall, bonds tend to (though do not always) go up.
If you are investing for 30 years, the risk of putting money into bonds is that you will not make as much money as if you had put the money into stocks. Historically (in the US or Canada), you'd have seen positive returns, just not as high as investing in the stock market.
There are many investment strategies. I live in Canada and personally favour the one described in the Canadian Couch Potato, a passive index investment strategy where I invest my money in Canadian, U.S. and International equity (stock market mutual funds) and also in a Canadian bond fund. There are, of course, plenty of people who will tell you to take a radically different strategy with your investments.