Here we go again.

Twenty-one months after China's Chengdu aerospace firm unveiled its J-20 jet fighter prototype – Beijing's first stealth warplane – the rival Shenyang company has revealed what appears to be a competing, radar-evading plane.

Over the weekend photos of increasing resolution leaked online depicting a previously unknown, black-painted warplane with the distinctive qualities of a stealth design. Perhaps it's only a coincidence that the stealth jet was revealed right before U.S. Secretary of Defense Leon Panetta was due to arrive in China. But the Beijing government is known to use these online leaks to show off its military advancements.

So China now possesses two potentially combat-capable stealth jets. But – and we can't emphasize this enough – it's not at all certain that either will make it through development, testing and full-scale production and into front-line service. Just ask the U.S. Air Force, which since the 1980s has overseen creation of no fewer than four different stealth fighter prototypes, but so far has only managed to equip just six war-ready squadrons with fewer than 200 operational jets. And at an extremely high price: up to $700 million per plane, depending on how you count.

The J-21 that appeared this weekend is outwardly similar to the nearly 2-year-old J-20. Both have two engines, two tails, big trapezoidal wings and the sharp, faceted features of a radar-evading plane. In that sense the J-21 and the J-20 evoke America's first batch of stealth prototypes, the twin-tail, twin-engine Lockheed YF-22 and Northrop YF-23.

Those two planes flew head-to-head in 1991, vying for an Air Force construction contract. The YF-22 won and, 14 years, a major redesign and some $70-billion later, entered service as the F-22 Raptor. Ten years later the Pentagon ran a second competition pitting the Boeing X-32 versus Lockheed's X-35 – both single-engine stealth designs. Again, Lockheed won, and is today developing the F-35 into a combat-ready warplane, though painfully slowly.

It's unclear whether Beijing intends to compete the J-20 against the J-21 for a single acquisition program. It's equally possible both jets are meant for production. It's also conceivable that neither is – that they're both strictly test vehicles. "Feng," an analyst writing for Information Dissemination, believes Beijing can only afford to manufacture one of the new planes and will be forced to choose. But that's conjecture. As with any Chinese weapons initiative, among outsiders there are more questions than answers.

For example, just how stealthy is the J-21 – and for that matter, the slightly older J-20? Both share the general shape of the U.S. F-22. But American stealth design relies on more than shape. Special radar-absorbing materials, sophisticated heat-absorption systems, "silent" electronic gear plus extreme high speed and altitude performance all combine to give the F-22 its so-far unique ability to evade enemy defenses. It's hard to say whether China has mastered, or even attempted, those techniques.

Moreover, if the airplane revealed this weekend is the new J-21, then what exactly is the partially-disassembled, shrink-wrapped airplane photographed being trucked through Chinese cities back in June? When that plane first appeared, some observers thought it was the J-21 being shipped in pieces to an airfield for assembly and testing. But the differences between it and Shenyang's new prototype are too big and numerous for the two to be directly related. Whatever the June jet is, it remains mostly unseen and, to outsiders, entirely unknown.

In other words, China has just pulled the cover off its second type of stealth fighter. But it may already have a third in the works. And it's even possible one or more of them will eventually evolve into a useful front-line warplane.