In this semi-futuristic war history, some militaries looked at the ultra-high performance concrete used in bunkers and thought: "hum, the only problem with bunkers is that they can't move, let's fix that".
And decided to make air planes made out of thick walls of ultra-high performance concrete that are heavy and fly at "incredible" speeds of 50-100 km/h (31-62 miles per hour).
Since these flying bricks of aircrafts aren't flying as fast as air liners (mach 0.8), they could use more delicate radiation-absorbent materials around its surface.
Although concrete (and the other additives) are somewhat already considered radiation-absorbent materials, since this brick fly so slow, they could become essentially invisible to radar.
In summary, normally these flying bunkers fly really low to hide in the curvature of the earth, and on top of that, are covered in at least a meter or more of this material. But, these are very slow and normally the length of a bus, around 20-30 meters (65-98 feet) long.
(Don't worry about the logistics, their autonomy and so on, that's not the question.
There are a lot of types of radiation-absorbent materials with varying degrees of effectiveness, but I'm specifically referring to the pyramid sponges.
From the wikipedia article itself:
"Pyramid RAM. The grey paint helps to protect the delicate radiation-absorbent material."
They are really fragile and wouldn't withstand the airflow, but, if someone were to cover an entire aircraft with such material and not get it ruined, would it turn the aircraft invisible to radar?
I say that because these sponges are not meant for this function, they are used to absorb the radiation noise from the outside; And the real world stealth aircraft designs put a lot of care on its shape in order to have a really low radar cross-section.