Josefina Odriozola was a 14-year-old girl shopping in the market with her mother when German and Italian planes supporting the Fascist forces of Gen Franco closed in on the town.
"I remember it well," she says.
"We left everything in the market and went home. We lived just outside the town, but the bombing started and we were there in the main square. Three planes flew in full of bombs and then left empty. Bomb, bomb, bomb, bomb, until everything was burning."
Josefina is one of about 200 people, many in their 80s, who are still alive to describe what they witnessed on that day. Today, it is not the bombing that makes her most angry, but what followed.
"They burnt the city down with their planes and they denied they had done it - they blamed it on the Communists," she says.
"My sister was 13 years older than me and they told her that the Reds had destroyed Guernica. But she said: 'No, the Reds don't have planes.' And they said to her: 'You little Red, we're going cut all your hair off.' Why? Because she was telling the truth. We couldn't even say the truth about the attack."
That same concern with historical truth is on the minds of more and more Spaniards as the country marks the 70th anniversary. Spanish society is becoming more interested in knowing the full story about its recent history, from the Civil War to the death of dictator Gen Franco in 1975.
Jose Ortunez and his Guernica History Association have spent 30 years reconstructing the truth about what happened here in 1937. The forces of Gen Franco blamed the attack on their enemy in the Civil War: the Communist-backed Republican government.
Thanks partly to work by people like Jose, Spaniards know the truth, that the attack from the air was by German and Italian planes supporting General Franco.
Gen Franco wanted to terrorise the people in the Basque region, an area of strong resistance to his nationalist forces in the Civil War. For Nazi Germany and Fascist Italy, it was an opportunity to get some practice with a new form of warfare: strategic, aerial bombing of civilians.
No strictly military objectives were touched. Factories and bridges were left alone - civilians were the only targets.
Ironically for a town almost completely destroyed by armed conflict, Guernica, before the Civil War and afterwards, continued to be a major production centre for bombs and automatic pistols.
The figures for the number of casualties in the bombing are still disputed, but most historians think between 200 and 250 people were killed and many hundreds wounded.
The attack not only terrorised the people of Guernica. This methodical and well-planned destruction spread fear across Europe on the eve of World War II.
But today, Guernica is sending out a more positive message. Iratxe Astorkia, director of Guernica's Peace Museum, says the permanent exhibition of the museum aims to make the visitor reflect on three things: the nature of peace, what happened in Guernica 70 years ago and what happens nowadays with peace in the world.
The museum and its centre for investigation have converted Guernica into a world centre for peace studies and conflict resolution.
And for many Spaniards, Guernica is symbolic of the renewed interest in unearthing the truth about their own recent past.
"I think Guernica is a good example of not forgetting and trying to go further," says Ms Astorkia.
"More and more young people in Spain want to know about it. They lost their parents, their sisters their brothers and they didn't know much more than that."
Ms Astorkia partly blames the education system for ignorance about this period. Barely a few pages are devoted to Spain's Civil War in official school text books.
An estimated 30,000 people murdered during the Civil War still lie in mass graves. The government is preparing new legislation that will officially honour victims of the Franco regime for the first time.
Keeping the young informed
With survivors and witnesses of the bombing in their 80s, the challenge now is to convey the importance of Guernica to a new generation.
One witness who does a very good job of that, is Luis Iriondo. Seventy years ago, as a 14-year-old boy, Luis ran across Guernica's main square and found refuge from the bombs in a shelter.
Through a doorway is the wine cellar-like room where Luis found safety with dozens of others. He says it was completely dark and there was no ventilation, so after five minutes he could hardly breath.
As the bombs started dropping he says he was terrified and expected to be buried alive.
"This bombardment lasted for three, maybe three and a half hours," he says.
"You could hear the bombs and feel the hot currents of air being forced away by the explosions. I tried to pray.
"Finally it finished, and I didn't really know what had happened, I knew that it was a bombardment and expected houses to be in ruins. But when I left the shelter I could see that everything was on fire."
Incendiary bombs had destroyed three quarters of the town. Luis fled to the hills and remembers looking back and seeing the buildings collapse. He says when he sees images of the twin towers falling down in New York, it reminds him of that day seven decades ago.
Today, 84-year-old Luis thinks it's more important than ever to remember Guernica and its message:
"War doesn't solve anything," he says. "It just sows the seeds for more war. World War I led to World War II. The attack on Iraq - look where that's led."
Luis is an artist and talks in schools about his experience, encouraging children to paint what happened in Guernica. Back in Madrid, it is the artwork of one of the world's most famous painters that has helped bring Guernica's message to millions of people.
At the Reina Sofia Art Gallery, Pablo Picasso's Guernica is always surrounded by visitors, of all ages, both Spanish and foreign. But it was not always in the gallery.
Picasso would not allow it to return to Spain while the country was a dictatorship. For that reason, says the head of collections at the Reina Sofia, Javier de Blas, many Spaniards associate the work with their country's desire to be free of Gen Franco.
"It was a symbol of this construction of democracy," says Mr De Blas. "The whole world accepted that the country had recovered its political and social liberties in part because Picasso permitted the return of the painting to Spain."
For many, it is also a constant reminder of the truth that the Franco regime preferred to cover up.
"We're in an moment of reflection concerning everything that happened in our recent past," says Mr de Blas. "This painting continues to do transcendental things in order to bring us towards understanding the truth."
After the death of Franco in 1975, there was an agreement between the left and right of politics, not to critically examine the past.But as the country marks 70 years since the bombing of Guernica, things seem to be changing. Many Spaniards feel that their transition to democracy will not be complete until they take a closer look at their recent history.