Have B-25's and B-26's been converted for private business use?
IMDB:


  McCall's plane is a demilitarized Douglas A-26 Invader, a type used in WWII. 


Since the film was made in 1960, I imagine there were plenty of them around.

(The A-26 was known a the B-26 from 1948 to 1965, according to Wikipedia. The B-25 is easily distinguished because it has the twin tail.)