1929: Speed, Comfort and Safety
1930s: Speed, Comfort and Convenience
1935: The Trans-Southern Route
1940: The Airline of the South
1948: None Faster. None Finer. To and Through the South.
1950s: Hospitality and Service from the Heart
1950s: One of America’s Pioneer-Scheduled Airlines
1959: The Airline with the Big Jets
1966: The Best Thing That Ever Happened to Air Travel
1968: Delta Is Ready When You Are
1970: That’s the Delta Spirit
1970: The Airline Run by Professionals
1974: Delta Is My Airline
1980: Airlines Are the Same. Only People Make the Difference.
1984: Delta Is Ready When You Are
1984: Delta Gets You There
1987: We Love to Fly and It Shows
1992: Ready When You Are
1994: You’ll Love the Way We Fly
1997: On Top of the World
2005: Good Goes Around
Current: Keep Climbing
More info and images here.