A little known classic...
You have a choice of playing as either an aeroplane or a zeppelin, each with its own special attributes, the plane being small and manueverable and the zeppelin being tougher but slower. Players must play through each level to fight a miniboss and then a main boss at the end.
You travel through greatly varying and increasingly difficult levels with interesting and detailed scenery that leads inevitably to enormous battleships and machines that need to be taken care of. There are many power ups available that give the player additional firepower or more points that help you along the way.
The music is generally pretty good for the Genesis system and sets the mood particularly well in a lot of levels. One problem with the game itself is that when there is a lot going on on the screen there are some slow-downs but nothing that interrupts the gameplay.
If you're looking for a "pick up and play" sidescroller then Steel Empire is a great choice, highly addictive and full of interesting features but really easy to learn, plus just to get from start to end would take an average of about 2 hours(but with no save feature thats a good thing).
So if you happen to find this game anywhere and want to dust off your old Genesis then give it a try.