This letter above is from Otis L. Guernsey (to Alfred Hitchcock) was sent in 1957. Guernsey had been able to work the idea into a suitable treatment and gave the idea back to Hitchcock (most likely for a fee). It became the premise behind "North by Northwest".
This undated letter below is from Otis L. Guernsey (to Alfred Hitchcock) was sent to the director in the early 1950s. What this proves is that the basic premise for NORTH BY NORTHWEST (1959) had been in the director's mind long before Earnest Lehman even met the director. Lehman was able to turn the convoluted premise into an amazing screenplay with the director.