Found it. The name of the short is Cannon Fodder by director and screenwriter by Katsuhiro Otomo, who is best known as the creator of the manga Akira and its animated film adaptation.
In a walled city perpetually at war, everyone's livelihood depends
upon maintaining and firing the enormous cannons that make up most of
the city. Nearly every building in the city is equipped with a cannon
of varying size, able to fire huge artillery shells over the city
walls. The story is centered on a young boy and his father, who works
as a lowly cannon-loader.
The city is surrounded by clouds of smoke and dust provoked by the
shots fired by the cannons. Despite news of successful bombardment of
the "enemy city" by the local media, there is not any visual
confirmation that it is true, or even if there is an enemy at all.
In the end the boy comes home from school and hears a television news
reporter talking about the near-destruction of the enemy city. The boy
hops into his bed, saying that someday he wants to be the exalted
officer who fires the cannons, and not be a simple worker like his
father. As he sleeps, a blue light sweeps across the window. Is it
from the enemy or the city?
Through unusual animation techniques the illusion is created that the
film consists of one continuous shot or long take.
Written and directed by Katsuhiro Otomo. Music by Hiroyuki Nagashima.