Right now, I have very little in-game information about the planes in Strikers 1945 PLUS, most of which comes from loose translations of Japanese pages. I haven't played this awesome-looking game yet. Some planes, like the Flying Pancake, behave quite similarly to their Strikers 1945 II incarnations. For detailed information about the planes in Strikers 1945 PLUS, visit the Strikers 1945 PLUS FAQ at GameFAQs written by Mav. Some info contributed by lfo.

The American Lockheed P-38 Lightning was a presence in every major Theather of Operations in WWII. It was made famous in the Pacific Theater of Operations, especially during the Battle of Midway (the setting of Capcom's shooter 1943, in which you also pilot a P-38).

The P-38 returns in Strikers 1945 PLUS. It can stock up to 4 Supershots.

The American Vought F-5U Flying Pancake was designed to be a fighter plane capable of remaining in flight at very low air speeds. These airplanes never got past the prototype stage and never was involved in any combat. That's right! The F-5U is a real plane, as weird as it looks!

The Flying Breakfast Item is one of the best fighters in the game. It has quick speed and its subweaponry is a highly effective auto-aim laser that passes through multiple enemies. Its Support special consists of two Northrop B-35 Flying Wings shooting at enemies and providing you with temporary shielding.
 

The German Focke-Wulf Ta152 was developed for the Luftwaffe as an answer to the Americans' high-altitude bombers, like the Boeing B-17 Superfortress.

In Strikers 1945 PLUS, the Focke-Wulf's Plasma Mine Supershot can destroy enemy bullets. Fully powering up its Subweapon, the Antitank Rockets, gives the Focke-Wulf a lot of attack power. Its Support special is a giant mother of an A9A10 rocket, which rams through enemies and absorbs all enemy fire! 

The British Supermarine Spitfire is an all-purpose fighter craft that served during combat during WWII in the Royal Air Force. The Spitfire page @ Aviation History Online Museum contains many technical specs about this aircraft. 

In Strikers 1945 PLUS, the Spitfire inherits the same weaponry that the Mosquito used in Strikers 1945 II. Its Subweapon is a rapid firing rocket. The Spitfire's Supershot, the Royal Napalm, can be accumulated relatively quickly, and does substantial damage. The Spitfire is an easy-to-use plane, which is a great choice for beginner players.
 

This is a class of Japanese WWII carrier-based fighter craft. You can see a swarm of these aircraft attacking Pearl Harbor in the Disney movie of the same name. For information about the Japanese Zero-type fighters, visit Astro Boy's Zero Fighter World.

In Strikers 1945 PLUS, the Zero is a very slow plane. It has very powerful weaponry to make up for that, though. Its Supershot is the most powerful of all the planes.
 

The Italians developed the Fiat G.56 originally under the demands of the Germans. Only two were ever built.

In Strikers 1945 PLUS, the Fiat's Support special involves three German Junker Ju287 Bombers. You can indirectly control these Junkers by moving your plane; the Junkers will line up to your vertical position on the screen. Also, the Junker formations can vary. 

The Curtiss XP-55 Ascender* is an American experimental fighter plane. This plane never got past the experimental phase; planes already in service outperformed the Ascender. Consequently, the Ascender never saw service. The name "Ascender" was meant as a joke by one of the Curtiss engineers. The name stuck and eventually became official. It looks somewhat like the Shinden J7 from Strikers 1945 and Strikers 1945 II. It even bears resemblance to the Chaser (Raiden Fighters) and the Chaser 2000 (Raiden Fighters 2/Jet)!! 

(Aviation Terminology: Planes built like the Ascender, Shinden J7, and the Chaser series from RF are known as being built in the "canard configuration." The tiny wing-like things near the nose of the aircraft are called canards, which help stabilize the aircraft. All canard aircraft have their engines in the rear.)

In Strikers 1945 PLUS, the Ascender is a secret plane. To select the P-55 Ascender do the following on the plane select screen: highlight the "?" Random Selector and perform this joystick sequence:  UP-DOWN-UP-DOWN-UP-UP-UP-UP-DOWN. Now, make a selection and you will play as the P-55 Ascender!

* (ed. note: this is possibly the only old link i've come across that isn't broken, holy shit!)