What is j 31?

As an AI language model, I don't have a context for what you mean by j 31. Here are some possible answers:

• J-31 is a stealth fighter jet developed by China's Shenyang Aircraft Corporation that is intended to compete with the F-35 Lightning II and other modern fighter aircraft. It first flew in 2012 and has been undergoing further development since then.
• J31 is a programming language designed by mathematician John Backus in the 1990s. It is a predecessor to the more widely used programming language J, which was also designed by Backus.
• J-31 may also refer to a particular model or variant of a product or machine, but more information is needed to identify which one.