It seems from all the articles I read on the topic that Knuth actually designed the font from ground up. He's a celebrated computer scientist, but was he also a digital calligraphy genius? That's a little astonishing to my mind.
"Design" is a loaded term.
Knuth knowingly and intentionally copied the metal font Monotype Modern 8A, which was the font used to set the first edition of volumes 1-3 of The Art of Computer Programming.
He described the procedure he started with in his lecture on receiving the Kyoto Prize -- see especially page 6, near the top. (A google search for "knuth kyoto prize lecture" turns up some other interesting links as well.)
This is really a transcript of the text, not a "finished" article. The figures are collected at the end rather than inserted where they are referenced. This lecture is also the leadoff item in Knuth's book *Digital Typography" (which is recommended reading in any event), where it is polished for publication, and the illustrations appear in context. (A discount for this book is available when ordered through the TUG bookstore.)
The important features of Computer Modern / Monotype Modern 8A are reflected in the design of METAFONT -- and that is truly a Knuth design.