In the Manhattan project they originally planned to use plutonium for their gun-type bomb and gave the design the name "Thin Man". However it was later determined that reactor-bred Pu-239 would inevitably be contaminated with Pu-240 whose high spontaneous fission rate would have caused a gun-type bomb to predetonate and fizzle, releasing only a small portion of its energy, unless the two pieces of plutonium were to be brought together at impractically high speeds and would have required a barrel too long to fit in any bomber. As such they switched gun-type bomb development to using U-235 which would eventually result in Little Boy while the use of plutonium was confined to implosion-type weapons and was the fissile material used in Trinity and Fat Man.
The logic behind the unusability of of Thin Man was that the Pu-240 impurities had a high spontaneous fission rate and produced fast neutrons with every spontaneous fission. These neutrons could then trigger a nuclear chain reaction when the two pieces of plutonium where near enough to constitute a critical mass but before they had reached their optimal position. The energy from this reaction would blast the two pieces of plutonium apart and quickly end the reaction before more than a tiny fraction of the plutonium had undergone fission, producing a fizzle that would have released an amount of energy impressive by the standards of chemical explosives but far below the kilotons of a proper detonation.
However U-235, the material used in Little Boy, also undergoes spontaneous fission and releases neutrons, albeit at a much lower rate as it has a much lower probability of decaying by spontaneous fission and a much longer half-life making decays of any kind much less frequent than in plutonium.
My question is, was it possible for Little Boy, or for that matter any U-235 gun-type weapon, to undergo the same type of predetonation and fizzle that made Thin Man nonviable if they were unfortunate enough to have a U-235 atom undergo spontaneous fission during the window where the two masses of uranium are close enough to sustain a chain reaction but have not reached the configuration where such a reaction would split the most atoms and release the most energy?