Couldn't do more than a 3/5 and here is why ...
While the game play worked alright, it really seemed to lack any depth. There are no levels, no goals, just a never ending flight as far as I can tell and I killed 6 or 7 of those flying fortresses.
The only 'power up' available is an armor restoring boost you get for killing the FF. It's like a bad version of Raiden, a little more updated graphics but worse game play.
My advice is to create levels of increasing difficulty. Add in some way to power up your plane. Maybe a hangar you go back to between levels or even just various weapons upgrades that drop randomly in game. If you want to go all out, create missions to accomplish this goal of saving your country, use the hangar for upgrades, maybe you can purchase new planes down the road, get money for kills, add in achievements.