Colossus was the first successful electronic digital computer that was used from 1944 onwards for decryption of messages sent by the German high command. Earlier than that, the cryptanalysts of Government Code & Cypher School (Bletchley Park) used "Heath Robinson" - a relay based device with electronic counting circuits, which was built according to Newman's specification. But it was slow and unreliable.
So, on Turing's recommendation Newman approached Flower, who worked almost independently with little official encouragement from GC&ES and built Colossus I. Ten Colossi were built but the later ones were advanced than the prototype Colossus I. The later Colossi had a special attachment, which enabled them to determine the patterns of the encoding wheels, and they contained 2400 vacuum tubes whereas Colossus I had only 1600. However, Colossus had two vital flaws - it had no internally stored programs and was not a general-purpose device.
Very few knew about the birth of Colossus and it was largely believed that the first fully functioning electronic digital computer to be built in the U.S. was ENIAC. The scientist duo J. Presper Eckert and John Mauchly of Pennsylvania University completed ENIAC in 1945 and this was larger and more flexible than Colossus. It is mention worthy, that John Von Neumann was a member of the ENIAC group and in 1945, when ENIAC was still under construction, and Neumann produced a draft report regarding the creation of EDVAC. But that is to be discussed later.