According to Guardian correspondent Julian Morruzi, the Guinness Book of Film Facts and Feats (Second Edition) states that the earliest pre-credit sequence can be found in Destry Rides Again (1939). Maybe so but Henry Hathaway‘s The Desert Fox (’51) contains the first lengthy (around six minutes) pre-credit sequence with a fair amount of dialogue — roughly similar to the length of a typical pre-credit sequence in the Bond films.

Incidentally: The brisk military drums heard over the opening credit sequence in The Desert Fox (6:20 mark) almost certainly inspired a similar drum track heard at the beginning of The Longest Day (’62), around the :12 mark.