Your'e right, the P39 would have the engine behind the cockpit with the exhaust stacks behind the greenhouse. It was an odd design as a mid engined fighter. It was not too successful as a fighter but found limited success as a ground attack plane. A lot of them were lend-leased to the Russians as tank busters early in WWII. I believe there are only two left in existance in airworthy condition.
see link:
http://www.warbirdalley.com/p39.htm