It was May 25th, 1938. Bertran, then just 15, was heading to the countryside outside Benasal, a town of around 1,000 residents in the eastern province of Castellon where a mountain range separates Spain's Mediterranean coast from its central plains.
"Suddenly three planes flew by, not very high. They turned towards the town and nosedived," Bertran, now 93, recalls as he sits in a rustic wooden chair in the living room of his home.
"They lined up and dropped their bombs. They fell very quickly, making a loud whistling sound. Within seconds you could only see dust."
He stops talking and thinks for a moment before adding in a broken voice: "When I returned to the town, everything was destroyed."
Photographs from the time show entire blocks of Benasal reduced to rubble, the dome and roof of the baroque church blown open.
At least 13 people were killed, victims of a new war tactic: air bombardments.
Spain's 1936-39 civil war pitted an elected leftist government against right-wing forces that rose up under General Francisco Franco, who went on to win and presided over a nearly 40-year dictatorship.
It was the first war where "aviation played a crucial role", said Barcelona University historian Joan Villarroya.
Planes bombed systematically the battlefront as well as the civilian population to "cause terror and break morale", he said.
Hospitals, schools, theatres, markets and even churches became military targets.
Historians estimate that at least 10,000 people were killed across the country in the air raids during the 1936-39 war.
The vast majority of the dead were opponents of Franco's forces, who were backed by Hitler's Germany and Mussolini's Italy.
The soldiers loyal to the Socialist-led government known as Republicans received help from the Soviet Union, but it was much more limited.
In November 1936 Madrid became the first European capital to be bombed by planes.
The following year the town of Guernica in the northern Basque region was wiped out by aircraft from Hitler's "Condor Legion" sent to Spain to support Franco, an atrocity immortalised in Picasso's haunting anti-war masterpiece named after the town.
At the same time Italian aircraft based in Mallorca bombed Spain's Mediterranean coast, especially Barcelona where 2,500 people were killed.
Spain was for them "a test ground for World War Two," said history professor Josep Sanchez Cervello of Tarragona's Rovira i Virgili University.
"They wanted to see what would be the effect of bombs on the civilian population. It was absolute panic."
Benasal suffered one of these experiments, the testing of the Junker-87, or Stuka, a German dive bomber that served the Axis forces in World War Two.
For decades no one explained why Benasal was targeted. It was an unimportant town, without troops and 30 kilometres (20 miles) from the nearest front.
But in 2011 Oscar Vives, a university professor who lives in Benasal, found a German military report titled "Images of the Effects of 500 kilogramme bombs".
The report proved that Benasal and three other nearby towns were used to test the dive bombers.
At least 40 people died "because of an experiment, of weapons testing," said Vives.
Time has not erased the memories.
Now aged 90, Rosa Soligo says she was in bed when the bombs landed near her house.
She recalls hearing her mother scream and "a loud noise" as part of the building came crashing down.
"When they pulled us out of the rubble our bodies were covered in blood because of the injuries but fortunately they were not very serious," she said.
The German dive bombers returned three days later but there were no longer any inhabitants left in Benasal. Everyone had fled.
'Punished by history'
"We lived in caves for days, for fear that they would return. We suffered a lot... a lot," Soligo said.
The effects of the air bombings can be seen still in the town of Corbera de Ebro in the northeastern region of Catalonia near the Ebro River, the site of the bloodiest battle of the war which paved the way for victory by Franco's forces.
Corbera was "completely levelled" by the insurgents, said professor Sanchez Cervello.
The town was engulfed in fire and smoke for weeks, and was called "the eternal flame", said local historian and high school teacher Joan Antonio Montana who provides tours of the ruins of Corbera.
Only the bell tower and facade of the town's baroque church survived.
After the war the surviving residents moved down the hill and rebuilt their town. The original town was left in rubble as a memorial to a "town punished by history".
By Daniel Bosque / AFP