Very likely custom/hand-drawn, but based on something like Gothic Extended:
http://www.myfonts.com/fonts/woodentypefonts/gothic-extended/

You can find a typeface based on the logo here:
http://www.dafont.com/search.php?text=PANAMERICAN&q=PanAm

The small scale of your image does not show how the letters are formed. This is clear in the following Pan American World Airways wall sign from the late 1950s and early 1960s:
Note the very geometric triangular [Latin} serifs:
The PanAm font is very inaccurate.
To recreate the lettering you would do better to start with a sans serif font which matches the letterforms and width.
Don

Shortened version of their logo freely available to download as vector here. As Kevin and Don's said, custom (and not a good one). I'd probably start with a contrasted wide sans like the ones suggested by Kevin simply adding those (ugly) latin/triangular serifs. Some more candidates for customization: Brenham, Condor (less fat/vintage/wooden type but more work on /R leg and /C shape)

Can't find anything closer than Gothic Extended or Brenham, which look almost identical.
The vector PANAM runs the letters together. Good find, but I would rely on the dimensions in the raster image I posted, particularly for the serifs.
Don

The secret behind the triangular Latin serifs.
The designer was at a loss how to update the old italic speed style logo.
As each idea failed he folded the sheet into a simple paper airplane, threw it across the room and had another shot of his favorite tipple. When he awoke he forced his eyes open and saw all these little white triangles scattered around.
Big idea! Paste triangular serifs on an old style thick and thin font and his job was done! He hoped it would fly with his boss just as well as his folded paper airplanes had done. An so it did. The little white triangles flew until PANAM went bankrupt in the 1990s.
Don

Thanks Don, for sharing such entertaining secret! :)
If one is more a cut person than a paste one, a reasonable approximation could be achieved pruning excess serifs from Latin Wide (the main problem being Latin Wide is a bit less contrasted than the Pan Am logo).

Can't agree more Don. "Type is weak and not really inspiring. It lacks feeling of 'speed', 'velocity', 'quickness', 'air', 'power', 'man' and 'conquest'. Why not making it Italic and add some light rays to each letters? And please, don't be afraid to "Make My Logo Bigger™". Everyday food for designers...

Loved your description of the process Don - good one!

Thanks for kudos. Perhaps would work well for an episode of _Mad Men_ :)
Don