These are some old DIY crossbow plans from the 1945 edition of "The Boy Mechanic". Some of it is outdated, but I'm sure there is a way to fix that. :) The metal on the front of the smaller crossbow could be replaced with something else, such as a thick strip of plastic. Anyway, I just thought this might help some of you people wanting to build a crossbow!
I made a few PDF files of this, and you can find them at (link removed). None of them are perfect, and the best one is turned sideways.
UPDATE 7/26/13 - Two years and 13,000 views later, I've finally realized that these plans are somewhat popular. I've updated the Instructable slightly and moved the location of the PDF files from the previous wiki to the Instructable itself. However, since I see no reason to take it down, the wiki will remain live for anyone who wishes to download or view the files that way. Also, thanks to El Supremo for providing an alternate download from 4shared (link removed). This PDF is also available as a download from the Instructable. Many thanks for this high quality download!
UPDATE 11/22/17 - It's been a good while since I've looked at this page... Links have been removed, as they no longer work. I have also looked back at the original book I scanned this from, and realized (thanks to a commenter) that I managed to skip two pages of the original article in my scans. I have therefore removed the incomplete PDF documents, leaving only the complete PDF originally provided by user El Supremo.