According to some official U.S. history, namely
http://www.centennialofflight.gov/es..._nav/POL14.htm
http://naco.faa.gov/index.asp?xml=fioo/fihistory work began on an ILS in 1928, and the first successful landing using an ILS was conducted by a certain Lt. James Doolittle on Sep. 23rd, 1929. That makes the idea contemporary with the Radio Range.
It took a little while to refine. The first "modern VHF ILS" installation was demonstrated to the U.S. military and airline industry in 1940 (second ref above).
My copy of Kayton and Fried is at the Uni, so I don't know what they say.
PBL