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.