Saturn family of rockets were designed to launch heavy payloads into orbits. After 25th of May, 1961, who conceived the idea that various Saturn vehicles can be put together in series to create a vehicle which can launch humans to the moon?
I suspect this is a duplicate. But here are two quotes from Chariots for Apollo by Courtney G Brooks, James M Grimwood, Loyd S Swenson Jr (this seems to be online here although I didn't discover this until I'd typed the below from my physical copy).
Chapter 2 is what you want to look at, from which page 49, bottom to page 50, top, my interpolations in [square brackets]:
On 16 November , Webb and McNamara reviewed the areas explored by Golovin's group and made several policy decisions. [...] And they decided that the Saturn C-4 should be developed for the rendezvous [as opposed to direct ascent, which would require a really vast vehicle called the Nova] approach to Apollo.
The C-4 was a rather smaller version of the C-5 (which became the Saturn V). From p47:
Configuration: booster stage (four clustered F-1 engines with 26.7-million-newton [6-million-pound] combined thrust) and a second stage (four J-2 engines with combined thrust of 3.6 million newtons [800,000 pounds]). The C-4 was briefly considered but rejected for the C-5.
Then from page 58:
On 4 December 1961, Holmes learned that Seamans essentially agreed with the committee's recommendations. [...] At its first meeting on 21 December, the council voted to develop the Saturn C-5.
The C-5 (p47 again):
Saturn C-5 (renamed Saturn V). Configuration: S-IC booster (five F-1 engines, clustered, with total thrust of 33.4 million newtons [7.5 million pounds]), S-II second stage (five J-2 engines with total of 4.5 million newtons [1 million pounds]), and S-IVB third stage.
'The council' was The Management Council which consisted of:
- Holmes, Low, Rosen, Charles H Roadman, William E Lilly, Joseph F Shea from headquarters;
- von Braun, Eberhard F M Rees from Marshall;
- Gilruth, Walter C Williams (Manned Spaceflight Centre).
The people mentioned by surname only have already been introduced in the text, but I was too lazy to grovel through it to find their full names: sorry. It should be easy in the above online version of the text.
Any infelicities in the above are because I can't type: check it with the online one.