Lockheed Martin (NYSE:LMT) just might be the best investment in the entire defense industry.

About a year ago, Air Force General Charles Davis boasted that that Lockheed's new F-35 fighter jet would ultimately become the world's first trillion-dollar warplane. This week, Davis's successor in charge of the F-35 program, Marines General David Heinz, is predicting the program will be either much smaller -- or perhaps much larger -- than that.

It all depends on how you look at the numbers -- what the definition of a "trillion" is.

A trillion here ...
According to Davis, the U.S. was planning to spend some $300 billion to acquire nearly 2,500 F-35s, and another $650 million to maintain and operate the birds. $1 trillion, give or take. But to hear Davis's successor tell it, Lockheed's actual take from this project could be much, much more.

... and a trillion there
Heinz predicts that Lockheed will sell "more than 3,100 F-35's to the U.S. and its allies. Other overseas buyers could account for 1,000 to 2,000 more sales as they buy F-35s to gradually replace obsolete Boeing (NYSE:BA) F-15s, Lockheed F-16s, and other jets currently in use around the globe.

So 5,100 warbirds in all. Now let's crunch some numbers. According to Lockheed, the runaway success of its new warbird is providing economies of scale so great as to drive the "fly away" cost of the F-35 down to as little as $83 million apiece -- and perhaps to less than $80 million. If we assume this best case scenario, then 5,100 planes times $80 mil each works out to a little over $400 billion.

Which is less than a trillion, right?
Sure. But that's just the "fly away" cost. Once you've "flown" the F-35 away, you still need to service and operate it -- creating recurring revenue streams for Lockheed. If we take Davis's assertion that $300 billion in sales equals $1 trillion in total revenues, therefore, then logically, $400 billion in sales equals ... perhaps $1.3 trillion in total costs, much of which goes to Lockheed. (And for its partners Northrop Grumman (NYSE:NOC), General Electric (NYSE:GE), and United Technologies (NYSE:UTX) of course. Don't forget about them.)

That's the big picture view on this project. More immediately, Lockheed believes that within the next five years, it must ramp production to the point where it's churning out one-plane-per-day just to keep up with demand. Assuming a five-day work week and $80 million a plane, that translates to around $20 billion per year in revenues -- six months of Lockheed's usual annual revenues, all from just one product line. And even working at full capacity, round-the-clock it would take Lockheed 20 years to build all the F-35s its customers require.

Foolish takeaway
Wow. Just wow.

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Fool contributor Rich Smith owns shares of Boeing. The Motley Fool has a disclosure policy.