When MEDLINE started it included information dated back to about 1966. In the 1990's the National Library of Medicine began adding additional years to its coverage, the OLDMEDLINE project. As of March, 2010, the database has now expanded back through 1947 and has reached over 2 million citations.
For more information see this article:
PubMed® Extends Its Reach — Biomedical Database Moves Back in Time to 1947. NLM Tech Bull. 2010 Mar-Apr;(373):e8. http://www.nlm.nih.gov/pubs/techbull/ma10/ma10_oldmedline.html