How was the little boy bomb triggered?

Answer 1

It used 2 pieces of 93.5% enriched Uranium-235: the target and the bullet at opposite ends of a cannon barrel. Cordite propelled the bullet down the barrel to the target. Once at the target, creating a supercritical mass, the bullet crushed the neutron source which generated a pulse of neutrons (about 10 in the time to set off the bomb).

For more details see The Making of the Atomic Bomb by Richard Rhodes.

Answer 2

It used a fission trigger. It goes something like this; The explosives fire and propel the bullet down the barrel, the bullet strikes the sphere and generator, initiating the fission reaction, the fission reaction begins and the bomb explodes.