H1B visa transfer on different title

I am having a H1-B stamping done from Company A but never went to USA on that Visa. Now I have joined Company B.

So, is my visa still valid and if yes, what is the process of getting it transferred to Company B?

B will have to file a cap-exempt petition (aka H-1 transfer) for you. Once approved, you can travel to work for B. If the visa stamp in the passport has expired or if B insists, you will have to appear for visa stamping w/ B’s approved petition.