When, where and which countries developed the most powerful military aircraft?
Well there isn't really an answer on your question.
There aren't fixed country's producing weapons. In history there were a lot different weapon factory's from state-owned weapon factory's to private weapon company's.
Who is responsible for that? The one who said he can develop weapons and take an offer from someone who want weapons. A link for this from Wikipedia.
To get better answers you have to specify the time range, maybe the war and which country you mean.
New answer (OP changed question).
The first munition was developed from 1250A.D. in china and later from 1324 in Europe, after the invention of black powder. In china they first used bamboo sticks which shot something out. Later in Europe they first built cannons mostly local at the beginning, cause the transport wasn't really easy.
The first weapon was a rock or stick used by one proto-human to bash another over the head...
And we've been getting every better at it ever since.
No country needed, though with the cost of large scale weapons development in the 20th century (or really from the Roman era) it certainly helps to have the resources of a country at your disposal.