In the arcade version and the Raiden Fighters Aces port you can select a hidden ship called the "Slave" (it's the ship you get with the S powerup) by highlighting a ship and pressing a button combo, then it selects the Slave with that ship's speed and bomb type (for example, highlighting Judge Spear and pressing the button combo will get you a Slave ship with fast speed and the bomb from Judge Spear). The Aces port had a different way to unlock the Slave, but in this version I'm told you cannot get it at all? Seriously? It doesn't detract from the gameplay in any way but as a port I'd expect it to have this feature enabled in some fashion.

I was giving Raiden Fighters Jet a go yesterday for the first time and when I chose the Raiden Type-II ship I got the slave ship.

I actually thought it was a bug. I just don't remember doing anything out of the ordinary on the selection screen.