The Fleet Line (as it was originally to have been called) was created in the 1970s, although the section north of Baker Street was already in existence as the Stanmore branch of the Bakerloo line. The first phase of new construction was a deep tube line from Baker Street to Charing Cross - a "new" station created by linking two nearby existing stations: Strand (Northern Line) and Trafalgar Square (Bakerloo Line)* - via Bond Street and Green Park. The line was given its definitive name in honour of the Queen's Silver Jubilee in 1977 (which also explains the silver-grey colour of the line on the tube map), in which year it opened.
The original intention had been to continue the Fleet Line eastwards from Charing Cross into the City (crossing the valley of the river Fleet, hence the name), but the changing patterns of London's development meant that this was never taken up, and in the 1990s the line was extended south of the river, diverting from the previous route at Green Park to go via Westminster and the main-line stations on the South Bank (with a temporary terminus at London Bridge pending further construction work), the new Docklands housing developments around Surrey Docks and then crossing the Thames three times to connect the Isle of Dogs - the commercial heart of Docklands, Greenwich (and the Millennium Dome) and lastly the Royal Docks area and Stratford, then a proposed location for the terminus of the Channel Tunnel high speed line and now a planned stop on it, which will also serve the site for the 2012 Olympics.
* This had the side-effect of requiring a change of name for the former Charing Cross underground station to its current name, Embankment.