There are two names to add, and one to correct.
A JPL summer-intern, Michael Minovich [now a Phd]
came up with the formalized concept of gravitational-assist
in 1961.
The Mr. Cuttings and Mr. Sturms [not Sturm] developed the concept of gravitational assist for a Venus-Mercury flyby mission.
There paper dates to 1964.
The material is found on Google, archived.
If you like reading scholarly material in the Adobe-Acrobat form, check out www.springerlink.com/index and surf on from there.
cheers.
Jacob