In Hitchcock's "North by Northwest", which actor is credited with the part of George Kaplan?

Advertising executive Roger Thornhill (Cary Grant) is mistaken for "George Kaplan". Kidnapped by two thugs, he is brought to the Long Island estate of UN diplomat Lester Townsend, and interrogated by spy Phillip Vandamm (James Mason). Vandamm's henchman tries to arrange Thornhill's death, but he manages to escape. Thornhill tracks down Townsend and finds he is not the man he met on Long Island. Townsend is killed instead of Thornhill, who is then wanted for murder. Thornhill flees and attempts to find the real Kaplan.

After incidents that take Thornhill from New York to Chicago, he begins to realise that his quest for Kaplan is not as it first seemed. When it looks like the thugs will pick him up again, he manipulates events so that the police pick him up first. The police then release him to the FBI chief (Leo G. Carroll), who reveals that Kaplan does not exist, and was invented to distract Vandamm from the real government agent…