Welter was born on 25 February 1916 in Cologne-Lindenthal. Following schooling and completing a merchant apprenticeship, he joined the Luftwaffe on 1 October 1934 and was trained as a pilot. Because of his skill as a pilot, he was trained as a flight instructor. During World War II, Welter was promoted to "Feldwebel" on 1 August 1940 and transferred to flight training school "Flieger-Ausbildungs-Regiment 63" and later on to the flight instructor school at Quedlinburg. Here Welter served as a flight instructor until 10 August 1943 when he was transferred to "Blindflugschule 10" (school for instrument flight training) in Altenburg with the rank of "Oberfeldwebel" as a night fighter instructor.