Interesting followup from Lew Galati:
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This was a unique plane the engine mounted behind the pilot. this allowed a cannon to be mounted so that it fired thru the nose cone on the prop. it was never used very extensively by the us because the p38 replaced it. it was better and so we sent the p39 to russia. It had an allison liquid cooled engine, same as many of the p38s I don't remember why it was not used by us but this design never went any where else. Only the ones they built and decided to send them to russia. I believe it was only built for political purposes to provide jobs for Bell.  It was touted as being special but it never lived up to the build up.

After 60 years, a Bell P-39 Airacobra was found intact in Russia with a Harrison Radiator in it.
Did you know that the girls, like Rosie the Riveter, who built these planes wrote their names and addresses on the plane's bulkheads?

http://link.brightcove.com/services/player/bcpid34757272001?bclid=0&bctid=87804472001