Actually I think it was the Martin boats that coined the name 'clippers' and it became applied to all the big Pan Am boats.
The first clippers were the Sykorsky one's... Starting with the S-40. the "Clipper" moniker invokes the relation with the fast trading sail Clippers of the previous century (XIX century, that is. There is even a poster juxtaposing a Boeing Clipper (the Yankee Clipper) with a sail clipper. Chronologically you have the S-40, S-42, then the Martin M-130 and then the Boeing B-314.
Reference:http://www.amazon.com/Pan-American-Clip ... tag=wix-20
(you can buy it with the amazon link in WIX
by the way, haven't been there but it seems a visit to the Passenger Terminal in Treasure Island is recommended to Clipper lovers...
@ Rádio Zero
My small blog of restoration volunteering
and, if you want pictures, see rreis@flickr