Don't look now, but Lockheed Martin (NYSE:LMT) just became vulnerable. And it happened at the company's moment of triumph.

Last week, as you probably heard, Lockheed wrapped up nearly a year of negotiations with the Pentagon regarding sale of a fifth batch of ultra-modern "Fifth Generation" F-35 fighter jets. In a deal worth an estimated $4 billion, Lockheed agreed to build 22 conventional F-35A fighters the U.S. Air Force, three short takeoff/vertical landing (STOVL) F-35Bs for the Marines, and seven F-35C carrier variants for the Navy. The Pentagon is paying extra, and negotiating separately with United Technologies (NYSE:RTX), for the engines that will be installed on the planes.

That should have been good news. On the surface, everything looks back on track for a project that Lockheed expects to generate upwards of $1 trillion in revenues for it over the next 60 years or so. And it would be good news, but for the fact that it wasn't the only big news to hit the wires last week...

Define "last"
A few years ago, then-Chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, described Lockheed's F-35 as being probably "the last manned fighter" that the U.S. ever builds. Created in an attempt to replace multiple mission-specific warplanes with one fighter than could "do it all," the F-35, and Lockheed's contract to build it, were supposed to secure the company's future for decades to come.

Emphasis on "supposed to." Turns out, that may not be the way things play out.

Last week, in a discussion with analysts held at the Center for Strategic and Informational Studies, Air Combat Command chief General Mike Hostage dropped a few comments that began to put the F-35's future in doubt.

This generation goes to "11"
Discussing future plans for the Air Force, Gen. Hostage mentioned that the Pentagon is already starting to look ahead to a "6th generation" fighter, due out sometime around 2030. The general noted that there are plans to create a new plane -- subsequent to the F-35 -- featuring "some kind of game-changing ability."

Will it be laser-guns? Warp drive capability? A cloak of invisibility? The general's playing mum for now, but just knowing that the Air Force is planning to build a sixth-gen plane within the next 15 years or so is enough to throw the F-35's future into doubt. It drops a strong hint that Lockheed's plane may not, in fact, be "the last manned fighter" the U.S. builds. It opens up the possibility that just a few years from now, Lockheed will have to gear up and compete again with rival plane makers like Northrop Grumman (NYSE:NOC), Boeing (NYSE:BA) -- against which it competed on the F-35 prototype -- or even a dark horse candidate like Textron (NYSE:TXT), which builds the V-22 Osprey in cooperation with Boeing, to build the Air Force's next superplane.

No more Swiss Army knives -- this is America!
For investors who thought that Lockheed's "competing" days were over -- that having won the JSX competition, Lockheed could just sit back and let the F-35 revenues roll in from here to eternity (or at least until UAVs put piloted fighter jets out of business) -- that has to come as a disappointment. And it gets worse.

You see, it appears the Pentagon is having second thoughts about the logic -- and cost -- of trying to build one plane to "do it all." Even at the Pentagon, a trillion dollars is considered a lot of money, and the cost of the F-35 just keeps climbing as time goes by. Now, talking about the hypothetical sixth-gen plane, Gen. Hostage says that he thinks what the military really needs is a "family of systems" -- multiple plane-types -- tailored to the different missions they'll be called upon to perform.

This suggests that far from trying to replace multiple F-15s, F-16s, and F-18s -- not to mention P-8s, A-10s, AC-130s, and similar specialized warbirds -- with a single do-it-all fighter jet, the Pentagon is starting to think that the more specialized, proven, and cheaper platforms in its arsenal still have a future alongside Lockheed's new F-35. And if that's the case, it's possible the Pentagon will wind up building far fewer F-35s than originally estimated.

It means that in the future, companies that build competing planes, such as Boeing and Northrop for example, may win more Pentagon business than previously thought, and Lockheed... less.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.