In 1943, during WW2, engineer Tommy Flowers designed and built Colossus, considered the world's first electronic digital programmable computing device, at Bletchley Park, to help solve encrypted German messages. After the war, Flowers applied for a bank loan to build another machine like 'Colossus'

...but was denied the loan because the bank didn't believe such a machine could work! His work on 'Colossus' was covered by the 'Official Secrets Act', and was not fully acknowledged until the 1970s. His family had known only that during WW2 he had done some 'secret and important' work.

Tommy Flowers recalled a crucial meeting between Eisenhower and his staff, during which a courier entered and handed Eisenhower a note, decrypted by Colossus at Bletchley Park. It supposedly confirmed Hitler was not moving additional troops to Normandy, this meant the British deception plan had worked, convincing the Germans the real attack was coming in Calais. Handing back the note, Eisenhower announced to his staff, 'WE GO TOMORROW'

Would  Eisenhower and Montgomery's D Day invasion have taken place

without the knowledge gained from Bletchley Park?