Campaign Usable Aircraft?

At what point in the campaigns are you able to upgrade your technology. I have started the campaign for the British and so far I am only able to use the Spitfire and the Typhoon. It is March 1944 and the front line has just moved across the border into Germany. We hope to have Berlin by Christmas. Would I be able to fly the Tempest or something else soon? I guess the question would be how do you improve the technology of your aircraft? For example, I have not yet encountered any ME262's in combat.
Thank you for the tips.
Thank you for the tips.