TL/DR: Let the dragons sneeze thermite.
Dragons are traditionally associated with "breathing fire", so rather than replacing their fire, we simply alter it a bit. Instead of exhaling a plume of high-temperature volatile substances that combust in the air, but travel slowly and quickly lose velocity, we give them the ability to produce a thick, sticky semi-liquid substance containing a mixture of chemicals such as magnesium and iron oxide, plus a pyrophoric substance such as white phosphorus. By making this liquid so thick and sticky that it behaves like a non-newtonian fluid, remaining cohesive even at high velocities, the dragon could take advantage of its long neck to inject globules of this substance into its airways as it exhales strongly, the fast-moving airflow accelerating the globules to higher velocities by directing the airflow through the nostrils which taper significantly along the length of the dragon's long nose. With an ability to produce high exhalation pressures and a significant reduction in the diameter of the nostrils from the back of the hard palate to their opening, it may be possible to eject these globules of thermite at velocities well in excess of those achieved by a human sneeze, that being claimed to be up to 90 m/s (320 kph / 200 mph). Velocities on the order of 250 m/s (900 kph / 560 mph) may be achievable, which is fast enough to hit all but the fastest of WWII aircraft from behind. Add the velocity of the dragon, which may be up to 30 m/s (100 kph / 70 mph), and this gives the thermite globules a velocity of between 220 and 280 m/s (800-1000 kph / 490-630 mph).
Given that a dragon has a long neck, it could turn its head to aim its thermite-sneezes in any direction, including directly behind itself, where a fighter aircraft would need to be in order to engage it.
What effect would these thermite globules have on an aircraft? Let us assume that the globules are roughly spherical, and around 20mm in diameter, containing a Magnesium & Iron oxide mixture, a sticky organic binder, and is coated with white phosphorus or some other pyrophoric substance to provide a source of ignition.
Magnesium thermite burns at a temperature in excess of 2200°C, easily hot enough to melt iron. Any globules that impact with an aircraft would flatten into a 'splat' perhaps 30-60mm in diameter, depending upon the impact velocity. The splat would be composed of either burning thermite or molten iron at a temperature of around 2500°C, hot enough to burn straight through the skin of any aircraft, be it doped linen or aluminium. Many WWII aircraft had wooden airframes, and those with metal airframes would be constructed from aluminium. Magnesium thermite would easily be hot enough to ignite both wooden and aluminium airframes, plus there is the significant probability that the aircraft's fuel tanks will be compromised, and the fuel within ignited. An impact on a glass or plexiglass surface would likely result in a burn-through, and an impact on an engine block, which would most likely be made from an aluminium alloy, could also lead to a burn-through which might result in a loss of lubricant and/or coolant which would result in the engine malfunctioning.
If a globule of thermite was to impact with a human pilot/crewmember, it would result in a severe burn that could easily be incapacitating.
A single hit to an aircraft might also have little effect other than to damage the aircraft's skin and add to its static drag, but as a dragon would likely sneeze many globules of thermite, multiple hits would greatly increase the odds of inflicting fatal damage.
The characteristics of the globules' combustion would also allow the dragon to aim them readily. Globules would initially leave a trail of white smoke as the white phosphorus burned, and then when the thermite core ignites, the globule would emit a bright white light. Both effects could be used by the dragon as tracers, allowing it to correct its aim to achieve a hit.
While the dragon itself would be armoured, and its hide might deflect a glancing impact from a bullet, achieving a hit on a relatively slow-moving target that is capable of sneezing thermite at 800 to 1000 kph in any direction would be both difficult and highly risky.