You can create your tunnel like so:
ssh -L 1521:server_b:1521 -N server_a
The first 1521 is the local port you want to bind to, so you can change that as you want. Then in sqldeveloper connect to localhost:1521
(where 1521 is, again, the local port you chose).
Of course if you're on Windows you can create the tunnel with PuTTY. The Session host would be server_a
, and then you go to Connection → SSH → Tunnels and enter 1521 as your Source port, and the Destination is server_b:1521
. Make sure you press Add after entering the info, then you can open the connection.