Where Was the World's First Programmable Computer Created?

The Colossus - Block H (The National Museum of Computing)
Bletchley Park, Milton Keynes 

Listed: 2004
Grade: II
NHLE entry: Listing details for Block H 

Imagine a world without computers in which you couldn't communicate using email, mobile phones or social networking. From the outset, computers have had a huge impact on the course of history, but where was the first programmable computer born?

Hidden in leafy seclusion

One likely answer, the 'Colossus', is found at Bletchley Park in Milton Keynes. The machine now commonly regarded as the world's first programmable, digital computer evolved as an unintended by-product of the Allied intelligence-gathering process during the Second World War.

Bletchley Park was taken over by the Government Code and Cypher School in 1939, chosen because of its leafy seclusion away from the prying eyes of enemy aircraft. The site played a pivotal role in the war, decoding German intelligence signals and so allowing Allied forces to better predict and prepare for the plans and movements of German forces. The machines developed here helped Britain to win the war much more quickly than would otherwise have been possible without them.

The Colossi in Block H

The Colossus was developed in 1943 by engineer Tommy Flowers, based on plans by the mathematician Max Newman. It was designed to decode the encrypted transmissions from the German teleprinter Lorenz cipher. British code-breakers described the German transmissions as 'Fish' and the messages they intercepted, or 'caught', as 'Tunny'.

Quite a few giant Colossi, as they were collectively known, were required because of the volume of signals being intercepted: by the end of the war, ten of them were housed in Block H, in the large rooms on the south side.

This was the last significant building to be erected at Bletchley Park and is the very first building made specifically to house an electric computer. But despite its historic importance, Block H, constructed in mid-1944, was built to look distinctly unremarkable.

Single-storeyed and in the shape of an inverted T, it had reinforced concrete walls and a corrugated asbestos roof and was built to a standard Ministry of Works hut design. But the significance of what went on under that roof should not be underestimated.

Highly classified

After 1945, all information on the Colossi remained highly classified, and it is said that Winston Churchill himself specifically ordered their destruction. The secret of their existence and achievements was so well kept that it was only relatively recently that any mention of their role has been made in histories of the period.

Flowers and his colleagues received scant acknowledgement for their crucial wartime work or for their development of what is generally believed to be the world's first programmable computer. The official silence surrounding Bletchley Park ended in 1974 with the sanctioned publication of The Ultra Secret by F W Winterbotham, who had worked at the wartime facility.

Today, as you step into Block H - now The National Museum of Computing - you can see a Colossus that was rebuilt from scraps of diagrams, old photographs and the memories of some of the team who had invented it. The Colossi once produced a thundering noise, which might perplex modern visitors who are more used to virtually silent computers.

Both The National Museum of Computing and the Bletchley Park Trust (which has transformed the main historic areas of the estate into a heritage and education centre) tell the stories of the various blocks and huts, all listed by us, and of the code-breakers who once toiled there.