Mantis Flying Fortress

The Flying Fortress is the bosses appears in Stage 3 and Stage 4.

Stage 3
"I will break you like a toothpick!"                                    -General Mantis, approaching you with his fortress.

The Flying Fortress takes form of huge bombers with 6-rear facing propellers with 5 gun port (2 on it's rear, 1 in the center, and 1 in each wing). The plane has green paint job with white stripes at the end of each fin.

Stage 4
The Flying Fortress appears to be a refurbished version of the Stage 3 Flying Fortress. How General Mantis extracting the salt-corroded remains of the Flying Fortress from the depths of the ocean is unknown. It is crudely repaired with part of the plane is crudely reconstructed with heavy duct tapes, several wooden planks, and some metal plating. However, it gains 2 additional gun port form it's previous.

Stage 3
At the rear of the plane is 2 Cannons with 1 in each fin. The cannons fire 1 bullet at the time and it's only firing in fixed rear position.
 * 2x Rear Cannons

After destroying 2 rear cannons, 2 more armaments show up.
At each wing, there are a gun port that is a chain gun. It fires bullets rapidly and reload after firing for a few seconds. The gun aims at the player. At the center of the plane it's a gun port for a directional cannon that fire 1 bullets in 12 direction at the same time.
 * 2x Chain Gun
 * 1x Red Radial Cannon

Stage 4
These are the same rear cannons that are also used in previous stage (Stage 3), but with increased rate of fire.
 * 2x Rear Cannons

After destroying 2 rear cannons, 3 more armaments show up.
Same as previous one in Stage 3, but the turret is in fixed diagonal position. It is also have different sound effects when firing. Next to a chain gun is a gun port that resemble cannons when pop up. These aren't the one that fires bullets. Instead, it launches missile that are lock-on to the player. The launcher appears to have much more hp than the chain gun. At the center of the plane is the gun port for the Yellow Radial Cannon. The cannon fires a single bullets at hexagonal formation at the same time.
 * 2x Chain Gun
 * 2x Missile Launcher
 * 1x Yellow Radial Cannon

Stage 3
Prior to this battle, A transmission from the unnamed in-game announcer (apparently your commander) will appear, telling you that General Mantis is about to attack with his Flying Fortress, and you must put him in his place.

Bait the initial cannons into shooting slightly to the side, and then attack them head-on. Once you destroy them, the plane will speed up and the second stage will appear: two chain guns on the wings, and one large central turret. If you have unlocked laser, it is recommended to immediately burn your lasers and take out the central turret. Otherwise, the screen will soon become filled with bullets. However, since the central turret shoots bullets in all directions but doesn’t really aim, it is possible to dodge between the bullets. Destroy the central turret first, and then focus on the side weapons. Move to the side while they shoot so the bullets will fly off the screen, and then move in to shoot them. Once you destroy them, the plane will start to smoke and splash into the ocean.

Stage 4
The stage 4 boss appears to be a refurbished version of the Stage 3 Flying Fortress. How General Mantis extracting the salt-corroded remains of the Flying Fortress from the depths of the ocean is unknown. Maybe he used his soon-to-be-fought Mantis Gunship. Or, the helicopters from the Canned Dread fight in Sky Force Reloaded picked it up. Either way, he put on a lot of bandages.

The first part of the boss fight is the same: two turrets. Just take them out — you know what to do. The second part is more difficult: one radial turret, one string turret on each side, and then one seeking-air-missile on each side. Curiously, the string turrets do not shoot straight, but shoot in a strange spread.

Just like Stage 3, your priority is destroying the radial turret. If you have lasers, burn them and destroy the center.

Next you should focus on taking out the SAMs, as they are the most difficult defense. Stay at the bottom of the screen and try to stay still for as long as possible to prevent bullet spread. If you still have lasers, burn them to take out the defenses on one wing (SAM + turret).

If you don’t have lasers, systematically take out the missiles and then the turrets. After the weapons are gone, be careful! The bullets are hard to see and you may inadvertently crash into them and even die, due to low health after fighting the boss.

Stay close to the boss's burning carcass as it falls; it doesn’t release any bullets upon death and also maximizes your chance of eating stars.

Trivia

 * The Flying Fortress is resemble to the one in the original Sky Force. The plane has same gun position, but the paint job in the original sky force is camouflage paint job.
 * During the second stage in Stage 3, parachutes inflate over the ocean and drop downwards. The origin or purpose of these is unknown.
 * In stage 8, a wreckage of the flying fortress can be seen near an airstrip, both the original and the reconstructed.
 * The wreckage of the Flying Fortress also appear in Sky Force Reloaded at Stage 5
 * The Flying Fortress resembles a Convair B-36 Peacemaker, an American Heavy bomber that was used from 1948 to 1959.