Ask job A for an extension on your decision. I’ve had companies wait as long as three weeks for my decision, but average was a week, maybe two. If you feel your interview with job B went well, tell them that you already have a pending offer and you need to know a timeline of the remaining hiring process. If they liked you enough, there’s a good chance they’ll expedite you through - my current company flew me out the weekend after my phone interview and offered me the position right after my panel interview when I told both the phone interviewer and panel interviewers that I had other offers waiting.

If Job B takes their sweet time with their hiring process and you’re considering taking job A in the meantime, make sure you fully understand any fine print or repercussions in the event you left job A so soon - stuff like paying back any sign on bonuses, training material, etc.