According to
Florence King's
Reflections in a Jaundiced Eye, the term "First Lady" for the
wife of a
United States President is actually borrowed from
Engish royal/
noble titles. On occasions when a
King of
England was unmarried (no
Queen Consort), his mother was not living (no
Queen Mother/
Dowager), and there is no
Princess Royal (traditional title of the king's oldest
sister), then the highest-ranking
woman in the kingdom would be the wife of the oldest of the Royal
Dukes (the king's
brothers and in some circumstances
uncles). She would go first in processions and get other queenly privileges, so she was designated "First Lady of England," though she would more commonly use her original title of
Duchess. For example,
John of Gaunt's wife Katherine, Duchess of Lancaster, was the First Lady of England after John's nephew
Richard II's first wife died until Richard's second
marriage.
I have found references to the term "First Lady of England," notably at
http://www.swynford.force9.co.uk/johngaunt.htm, but I have not found any confirmation that the American title was chosen because of the English usage.