Turret equipped fighters became operational with the Royal Air Force during WW2 as the result of the British Government’s ongoing concern about German aerial attacks on the United Kingdom during World War One. The possibility of enemy bombers, and not just German ones, once again flying over the country and dropping their deadly load on British soil had troubled them ever since. In the 1930s a fighter fitted with a gun turret was considered to be an effective way of attacking bombers and would be a feasible alternative to conventional fighters. The result of this concept was the Defiant, which was developed to meet this operational requirement. After a promising start during the dark days of the Battle of France the Defiant was withdrawn from day operations, after having sustained heavy losses, during the Battle of Britain. This first part is dedicated to the Defiant and narrates its development as a day fighter. Although the concept proved a failure, the blame does not rest solely on that Defiant, the easy way taken by some writers in the past, as other factors contributed to its failure. Over 30 photographs, some seen for the first time.