Part 1 got us into our first battle, Part 2 covered the main mission types, in Part 3 we’ll have a look at buying new aircraft and weapons.
As you fight battles you should hopefully be gaining experience, and therefore ranks in your chosen air force(s), allowing you to buy new planes as we did with the Swordfish in Part 1. Looking at the British tech tree, Rank 1 unlocks the Gladiator, a slightly better biplane fighter, Rank 2 gets us into monoplanes with the Hurricane Mk I and Blenheim Mk IV, and Rank 3 upgrades those to the Hurricane Mk II and Beaufort Mk VIII. You can put these into service to replace your starting biplanes; if you have 50,000 spare silver lions (completing the tutorial missions is a good way of earning a few) you can also recruit another crew for a hanger of five aircraft. Further crew can be recruited, but at the cost of real money golden eagles; the tutorials should have earned enough to cover an additional crew, but you should be sure you want to stick with a country before spending them. Extra crews are most useful in Arcade Battles, where you can use your whole hanger, if you’re planning to focus on Historical or Realistic battles then they’re less important.
The new aircraft you unlock come in three basic flavours: the Hurricanes are Fighters, the Blenheim and Beaufort are Bombers, and there are also Attackers, that don’t show up for the British until later. (Update: since this guide was written the system has been heavily overhauled, so aircraft now have much more specific classifications; as well as just “Fighter” there’s e.g. “Fighter / Light Bomber”, “Heavy Fighter / Torpedo Bomber” etc.)
Fighters
Fighters, as the name suggests, are designed to fight other aircraft. Most fighters are single-engine single-seat planes like the Spitfire and Zero, but you also have heavy twin-engine fighters, Zerstörer in German terminology, typified in the early tiers by the Messerschmitt Bf 110; these tend to pack heavier armament at the cost of manoeuvrability. Heavy fighters are well suited to going after bombers, where their lack of manoeuvrability isn’t an issue, if you’re flying one it can be worth climbing at the start of an Arcade battle in case enemy bombers try and hide up at high altitude.
Many fighters can also fulfil a fighter-bomber role when fitted with a few bombs or rockets, if you get bored of shooting at flying things. These may need to be unlocked; see “Weapon Loads” below. Hanging bombs from an aircraft will slow it down, so if performance is a priority (like fighters in Historical Battles) then just stick to guns, but if you’re in a ground strike Arcade battle you might as well fit a bomb or two if you have the option, you can always just drop them if you find yourself in a frantic fight (just be a bit careful you’re not over friendly ground forces at the time).
Bombers
The main job of a bomber is to… anyone? 5 points if you said “drop bombs”. There are two main techniques, both covered in tutorials: level bombing from altitude using the bombsight (F7), and dive bombing. Level bombers, starting with light and medium types like the Blenheim and Ju-88 and eventually culminating in massive heavy bombers like the Lancaster and B-17 Flying Fortress, can generally carry more and/or heavier bombs, and have multiple gun turrets for defence which can be quite effective if an opponent just sits on your tail (especially if you man the guns yourself using F6, if you can afford to fly straight and level for a while), but as fighter armament gets heavier it’s not often a duel you can win. If you want a friendly fighter escort you’ll probably need to bring a squad-mate, team co-ordination is something of a rarity in Arcade matches, so one way of trying to stay safe is to climb as high as you can, especially in the early ranks when less powerful engines mean it takes a long time for enemy fighters to gain altitude; if the enemy team are all distracted in low level engagements you can drop your bombs and potter around in safety waiting for them to reload, but a couple of heavy fighters might have read the previous section and climbed up to meet you. Sorry about that…
High altitude bombing only really works against static targets, due to the time it takes bombs to fall; if there are ships or convoys of moving vehicles then you can line up for a low-level pass, but be prepared for a one-way journey, a nice fat bomber trundling along at low altitude attracts as much interest as a particularly impressive grapefruit spoon at an international cutlery convention. After you drop your bombs you can always switch to the rear turret and try and plink a few of the fighters who’ll inevitably attach themselves to your tail, and if you’re really lucky have a good laugh if they get over-eager and collide with each other.
Dive bombing is more accurate, if your aircraft is built for it; examples in the game include the Ju 87 Stuka and SBD Dauntless. If in doubt, have a look at the nice big bombing reticle on the ground in Arcade mode; when there’s a solid crosshair your bombs will land right in the middle, the more open the lines are the greater the dispersion, until eventually a diagonal line indicates bombs can’t be dropped at all. Level bombers are at their most accurate in level flight, dive bombers remain accurate while in a screaming dive towards their target; attackers and bomb-equipped fighters are only accurate in a dive at low altitude.
I like to gain a decent amount of altitude and fly slightly past a target (as long as there aren’t any prowling fighters), so after the attacking dive you use all the speed you built up to continue back towards your home airfield and (hopefully) comparative safety as fast as possible, rather than bleeding a lot of speed in a turn.
Attackers
Attackers are ground attack aircraft that operate at low altitude, armed with bombs, rockets and/or cannons. Few aircraft were designed specifically for ground attack, the Il-2 Sturmovik being the definitive exception, precise designations get rather blurred between heavy fighters, light bombers, fighter-bombers and such, so other aircraft classified as Attackers in War Thunder are a bit of a mixed bag like the D3A Val dive bomber, P-47 Thunderbolt fighter-bomber and Bristol Beaufighter heavy fighter with quite varying capabilities. A key difference between Fighters and Attackers, in War Thunder mechanics, is the bomb reload time in Arcade; the exact value depends on the crew Reload skill, but is roughly six minutes for the former compared to one minute for the latter.
If you really excel at one particular aspect of the game you might want to concentrate on a single type of aircraft, but generally it’s useful to have a mix in your hanger to allow you to fulfil different roles within an Arcade mission, such as ground attack at the start of a Domination map to clear out some targets, then switching to a fighter to defend the airfields. With a selection of aircraft of different ranks you can make tactical choices, like starting off in your lowest rank fighter and hoping that everyone else gets their better aircraft shot down, so you’ll have more of an advantage later in your better planes, but then if everyone else is doing that then maybe you’re better starting in your top aircraft to rack up some easy kills…
Weapon Loads
Some aircraft also have multiple possible weapon loads. Click the Weapons button (three bullets on the icon) to see the possible weapon selections; the Hurricane I, for example, can only ever have 8 .303 machine guns, but when you buy the Hurricane II at Rank 3 you get some more exciting choices:

The Hurricane Mk II can mount rockets or bombs in addition to its machine guns
As well as its machine guns, you can fit two bombs or six rockets. If you’re used to homing missiles in modern flight sims, by the way, these are unguided rockets, for air-to-ground attacks, although if you do happen to hit an aircraft with them (like a nice big bomber at close range) they will make a mess of it. You need to spend silver lions to unlock the options, but as they’re pretty cheap then unless you’re stone broke I’d suggest hitting the “Buy All Weapons” button. Once you’ve bought them, you can choose which to fit at the start of a battle or when you respawn:

Choose your weapon: sword, pistol or six 60lb 3-inch Rocket Projectiles
I often like to start a battle in a fighter-bomber; head for the nearest enemy ground units, rattle off the bombs or rockets, then look out for any friendly bombers who might need an escort, and see how the battle is developing when it comes to respawning.
The Hurricane II has the extra options available from the start, but other aircraft like the Beaufort have to unlock extra fittings to get new weapons:

The Beaufort carries four 250lb bombs by default, the options to carry a torpedo or two 500lb bombs are currently locked
Looking at the “Modifications” section, you can see that the TBC pylons required to carry a torpedo require 22,000 aircraft experience:

* batteries not included. TBC pylons must be purchased separately. Your aircraft may be at risk if you do not keep up repayments.
I must confess that I don’t know the precise mechanics behind individual plane XP compared to country XP, player XP and crew XP. You can see the current total at the top of the the window (zero in the screenshot above in my newly purchased Beaufort); basically the more you fly and better you do, the more experience you get. Keep checking back in and eventually you’ll be able buy the pylons to fit bigger bombs and torpedoes. Most aircraft can also upgrade their engines and reduce air resistance for better performance, with enough XP.
Update: Patch 1.29 introduced new ammunition loads for guns, allowing you to upgrade the default ammunition belts to include all sorts of fun like Armour Piercing, High Explosive and Incendiary bullets and shells. You need to unlock the options with Aircraft XP then buy them with silver lions, much like other Modifications.
Different calibre weapons of different nations have various options available; most have “omni-purpose”, a mixture of various ammunition types that should be suitable for all target types, and a “stealth” option with no tracers so that you don’t alert your target if you miss with your first burst (if you hit them, the big holes in their aeroplane are usually a bit of a giveaway). There are also some specific air and ground options, and a few other exotic choices like All High Explosive Incendiary Tracer, All The Time.
I’ve yet to find a definitive guide on the exact effects of the various ammunition types and the best choices, but the second half of this post on reddit at least explains the various acronyms and shows the different options.
While on the subject of weapon loads, a lot of bombers have the ability to carry a lot of light bombs or a few heavy bombs. The Su-2, for example, with the right pylons can carry 12 x 50kg bombs, or 6 x 100kg, or 2 x 250kg. Though, as per the old maxim, quantity has a quality all its own, I tend towards the other maxim that bigger is better. If you can get nicely lined up on a road packed with a column of vehicles it can be glorious to drop a long string of bombs down it, but irritatingly the enemy are seldom generous enough to lay out their forces in nice, geometric, easily bombable patterns. Larger bombs don’t have to be quite so precisely placed, particularly useful from high altitude, and some targets need heavy ordnance to destroy them.
Regarding weights, just to confuse people metric and imperial weights are used by different nations (e.g. 250lb bombs on the British Beaufort, 250kg bombs on the Soviet Su-2). 1kg = 2.2lb, so as a very rough rule of thumb you can multiply or divide by 2 to get a general idea of comparative bomb loads, or type a phrase like “500lb in kg” into Google and its handy-dandy converter will give you a more precise figure.
To wrap things up, Part 4 covers the most important feature of the game: painting your aircraft, along with some minor sections on crew skills, premium options and the like.