Answer: Colossus Mark 1

Colossus was the first programmable digital computer albeit a very limited one by modern standards–it had no internal storage for programs and was not a general purpose machine as it was built explicitly for crypotographic functions. Designed by Tommy Flowers in order to execute the code breaking algorithms designed by Max Newman, the computer was absolutely influential in helping the Allies win World War II.

Colossus Mark 1 was in operation by February of 1944 and played a critical role in helping Allied forces decipher encoded Axis transmissions. Because of Colossus, for example, Allies knew Hitler had no intention of sending reinforcement troops to their chosen landing locations at Normandy.

Bonus Trivia: The level of secrecy surrounding the Colossus project was so high that evidence of the program was completely suppressed until the 1970s.