The first transatlantic flight by aircraft was completed on 27th May 1919 by the NC-4. It took 19 days to finish the journey starting from New York State to landing in Lisbon, Portugal.
The aircraft had to stop several times due to repairs and for crewmen’s rest. It was, thus, not a nonstop flight. In fact, 2 weeks later the first nonstop transatlantic flight was made by John Alcock and Arthur Whitten Brown, pilots of the Royal Air Force, which eclipsed the feat of the NC-4 in the eyes of the public. FYI, the nonstop flight had taken almost 16 hours.