As we've seen the J-20 project proceed to the pre-production prototype stage, Project 310 (Shenyang AC's 5th gen design) is continuing its flight testing. Although, I and many other have called it J-31, it really hasn't become an official PLAAF project yet, so it has no J designation Many have called it the J-21 project, because they expect it to receive that designation once it becomes official.
There have been a lot of discussions online about where this project is at and how is it funded, so I will give me take here. At this stage, project 310 only has one flying prototype in No. 31001. Some would compare it to No. 2001 of J-20 project, but I consider more as a proof of concept aircraft similar to X-35.
For example, I think the pre-production prototypes should not longer see the gap between the engine nozzle and nacelle. From what I observed online, it seems to at least have received some funding from PLAAF to arrive at this stage, although SAC does has enough resource/funding to get here by itself. I think we are unlikely to see a second flying prototype until it becomes an official PLAAF project, since No. 31001 and a possible static prototype can give all the data PLAAF would need to make its decision.
Based on all I have read, it seems to be foregone conclusion that will happen. Once it does get designation and full funding from PLAAF, SAC is likely to make numerous changes on the next flying prototype and also start conducting radar and weapon testing.
Just looking from front, Project 310 has the contours that one would expect out a stealth aircraft. One does not need to look far before seeing comparison to F-35 and reading articles about "stolen technology" from F-35. Whether this jet will turn out to really be a stealth aircraft depends on all the little details that the designers have to look through to minimize returns from all around. Until we get to a later prototype, it's really hard to say how well Project 310 will do in that area.
I am generally not too concerned about the electronics on the new Chinese aircraft, because I think they have really made huge progress here. I think they can achieve comparable target identification and situation awareness as F-22/35, since it will come into service at a later point when newer technology will become available. I think the biggest concern for this aircraft is its engine.
This is a problem with all new PLAAF aircraft. Clearly, RD-93 will not be powering Project 310 in production. The 9.5t class "Medium Thrust" engine models have been around for several Zhuhai airshows. I would imagine that engine will be used on Project 310 and other projects like UAVs. Even though it has great important, it will not be as high priority as the WS-15 project.
Since it began at a later point and has lower priority than WS-15, it definitely won't be ready before WS-15. If we estimate that serial production of WS-15 will be under way by 2020, this medium thrust engine won't be ready for serial production until 2025. PLAAF would have to either wait until then or find an interim selection. A large part of its test flight program may need to be done with a different engine.
The other question is what is the expected market for Project 310 outside of China. Unlike J-20, models resembling Project 310 have appeared in air shows since 2011 indicating that it should be available for export sooner rather than later. The problem is who can they sell it to. Ten years from now, most of the Western countries will probably go for F-35.
The remaining ones will either go for the eurocanards or for super hornets if they are still in production. Due to politics, they are unlikely to go for a Chinese or Russian aircraft. We know that India and Russia are committed to the PAK-FA project, so what does that leave for Project 310? Most of China's traditional customers in Asia, Africa and Latin America simply don't have the money or the need for a low-end 5th gen aircraft.
Project 310 will most likely be exported to Pakistan. After that, it will have to battle against F-35, PAK-FA and Gripen-E in the Middle East, South East Asia, Brazil and South Africa. Its main advantages are its cost and available production slot, but China will have to move fast.