Every indication is that nobody in Western intelligence saw the Chengdu J-20 coming. While it was known that China was developing a stealthy combat aircraft, the J-20 has emerged earlier than expected and appears to be more mature than the X-plane or demonstrator that many people anticipated.

The debut of the J-20 had been predicted in a November 2009 interview on Chinese television by Gen. He Weirong, deputy commander of the People's Liberation Army Air Force. The general said at the time that a “fourth-generation” fighter would be flown in 2010-11 and be operational in 2017-19.

At least two J-20 prototypes were complete by the time the aircraft made its first flight—or at least its first public flight—on Jan. 11, 2011. The two aircraft are distinguished by the detail design of their exhaust nozzles, leading to speculation that one of the aircraft has Russian-supplied AL-31F engines, of the type fitted to the Chengdu J-10, and the other has the Chinese-developed WS-10 engine.

The J-20 is a big aircraft. Although its overall length (around 66 ft.) is not much greater than that of the 62-ft. Lockheed Martin F-22, the main structure from nose to exhaust nozzles is considerably longer. Like the F-22, it has large weapon bays in the lower fuselage and smaller side bays, the latter probably dedicated to air-to-air missiles.

The J-20 echoes the canard configuration of the J-10, but with canards level with, and immediately in front of, the wing. Two small, canted, all-moving vertical stabilizers are fitted. Although no U.S. manned stealth aircraft have flown with canards, a tail-first layout was featured by early Joint Strike Fighter designs, including Lockheed Martin's—which the J-20 resembles—and McDonnell Douglas' X-36 unmanned demonstrator.

Stealth design features mostly follow Lockheed Martin F-22 and X-35 practice. A high chine line around the forebody continues through the inlets and upper body, and flat, canted side surfaces blend into a flat underside via a small-radius edge. The canopy shape is also reminiscent of the F-22. The J-20 uses a diverterless supersonic inlet (DSI)—originally developed by Lockheed Martin, DSI technology is now used on the J-10B, JF-17 and (according to one report) the Saab Gripen JAS 39E/F.

The rear-aspect view of the aircraft is not as stealthy, a feature also seen on the Sukhoi T-50. This is clearly an intentional trade, eliminating the heavy 2D nozzles of the F-22. In this respect, both the T-50 and J-20 reflect the philosophy behind the pre-1986 Advanced Tactical Fighter studies that preceded the F-22, based on the theory that a fast, high-flying, agile aircraft is relatively immune from rear-quarter attacks.

According to a Chinese paper released on the Internet, the main goal of the design was to achieve high speed and maneuverability with the engines that would be available to China in the near future—the AL-31F and WS-10—which do not have the same thrust/weight ratio as the latest Western engines. This resulted in the selection of a delta wing and relatively long body for low supersonic drag, plus large, high-deflection canards to provide agility. The all-moving vertical tails are said to be 40% smaller than conventional fin/rudder designs, and accordingly lighter. Supercruise is probably not attainable with existing engines, but the design looks capable of it, once propulsion technology in China improves.

In 2012, China-watchers will be monitoring progress with the flight-test program and looking for signs of work on the many challenging aspects of stealth. A stealth fighter needs multispectral, active and passive sensors to detect and track its targets, and those sensors need to be fused and managed to minimize emissions. Similarly, to operate at maximum effectiveness as part of a networked force, stealth aircraft need effective low-probability-of-intercept voice and data communication systems. These are problems that the U.S. is still wrestling with, after 25 years of work.

There is another, more fundamental question: What is the J-20 for? The fighter is large for air combat—but China, simply because of geographical factors, doesn't face an adversary fighter force of the kind that the F-22 was designed to counter. At the same time, the J-20 weapon bays are not large enough for most standoff air-to-surface weapons. One possibility is that the J-20 is intended to threaten intelligence, surveillance and reconnaissance assets and tankers, by using stealth and speed to defeat their escorts.