The first advantage is that the J-20 became easier to design and build. An emphasis for head-on stealth has simplified the stealth requirements of the aircraft, not unlike what has happened with the F-35. Because the aircraft's primary mission is to strike targets without being detected, rather than engage in prolonged battles at altitude, the aircraft needed less emphasis on 'knife fighting.' This also simplified design.
The lack of a need for thrust vectoring also allows China's jet engine industry to take another year or two before being needed to provide the F60 (J-31) with a TVC-compatible engine. And, of course, all the lessons learned from the J-20 can be applied to the F60, allowing the air superiority fighter to exploit greater expertise than the strike fighter. Additionally, while thrust vectoring is intended for later variants of the J-20, its mission is not significantly compromised by the lack of them on the Block I aircraft.
The J-20's design, opting for a strike fighter emphasis rather than an air superiority emphasis, was driven by a number of realizations.
First, the PLAAF could never really hope to match the F-22 with its first foray into stealth aircraft. Second, only two other stealth aircraft will be available by the time the J-20 is unveiled, minimizing the risk of markets being inundated by competitors. Third, the simplified engineering allows a final production aircraft that costs less than its counterparts, potentially in the vicinity of $100 million per aircraft with export downgrades; this allows China to win the pricing point, which has long been its strength in the global arms market.
The choice to emphasize strike missions over true multirole missions, which the J-20 is theoretically capable of (and, indeed, it is billed as a 'multirole' fighter), was the result of a detailed assessment of Chinese strategic needs. In a conflict with South Korea, China could reliably expect cruise missiles to greatly compromise the efficiency of the ROKAF. This minimized the need of a stealth fighter on that front.
In a conflict with Taiwan, the PLAAF possesses such a comprehensive advantage in numbers that air superiority could be established over the island without the need of a new fighter -- though the newest J-11s are certainly intended to ensure dominance in such a conflict. India's move towards a new fighter has seen her emphasize 4.5 generation aircraft, further minimizing the need of a stealth fighter.
Finally, China's large littoral region -- the logical conflict point with Japan or America -- is relatively sparse in terms of air bases. The First Island Chain is firmly within range of current and future cruise missiles; efficiency in this area would be hard to guarantee. The Second Island Chain is significantly further away, complicating any long-range strike missions.
Geography complicates the deployment of the F-22 and F-15, both of which rely upon land bases, and would force the United States and Japan to make do with air coverage provided by aircraft such as the Super Hornet. While this is believed to be a superior fighter to the J-11B, it is not believed to be so significant as to overcome the significant numerical disadvantage it will face. Additionally, carrier deployment is complicated by the DF-21D.
The J-20's design reflects China's strategic needs. It is clearly geared towards deep strike missions, being optimized to avoid detection by American task forces until the last possible moment, at which point it would deploy a variety of air-to-surface missiles. To further facilitate the multirole intention, the J-20 will feature both single and dual seat variants.
The design will, of course, utilize secrets gleaned from the F-35 "leak" -- a particular fact that the Chinese government will be conveniently opaque on. The J-20's frontal stealth aspect will be roughly comparable to that of other sub-1m^2 aircraft.
The J-20 will also possess an AESA radar, though the current model under consideration is still in testing.
Specifications ;
Crew: 1 or 2
Length: 22m
Wingspan: 13m
Height: 4.5m
Wing area: 60m^2
Empty weight: 17,250kg
Max takeoff weight: 37,500kg
Powerplant: 2x WS-15
Maximum speed: Mach 2.0 (Supercruise compatible)
Range: 5,500km
Combat range: 2,000 km (1,243 mi; 1,080 nmi)
Service ceiling: 20,000 m (65,617 ft)
Internal Payload: One primary internal bay (under fuselage) with 2 hardpoints rated for up to 1000kg each; two auxiliary internal bays with 1 hardpoint rated for AAMs (intended for missiles in the 150kg weight class, but theoretically rated for the 310kg PL-11).
External Payload: 7500kg over 8 other hardpoints (one on each wingtip, three under each wing) for a total (stealthy and non-stealthy) payload of 10,250kg.