yes
I suggest reading Richard Rhodes book: The Making of the Atomic Bomb.
FDR
no, but they were involved as couriers to leak information on the bomb to the USSR.
The United States
Creation of the atomic bomb
well, the atomic bomb research was done by Robert oppenheimer. the original creation or idea was from albert Einstein
One was the discovery of Plutonium.
H.G Wells
Yes
J. Robert Oppenheimer, while working on the Manhatten Project, directed the creation of the atomic bomb.
Oppenheimer was the scientific director of the development of the atom bomb in New Mexico.
There are many different elements involved in the construction of an atomic bomb, but the elements that actually cause an atomic explosion are either uranium (specifically the U235 isotope) or plutonium.