Coal has a reasonable enough energy density to be used as aircraft fuel. The reason it isn't, and liquid fuels are used instead, is that solid fuels require very laborous processing and complex systems to produce the kind of even continuous combustion that liquid fuels can do with just a pump.
In peacetime, with oil as cheap as it is on the world market, it would never make sense to produce a coal-powered aircraft. At war, with no oil imports coming in, and in desperate need of operable aircraft, the investment of processing coal into even granules to burn in a ramjet might.
