In the UK at least, RAF pilots who have gone through basic flight training have a broad choice between fast air, widebody and rotary wing (helicopters). Once you pick a particular type (and assuming you pass the relevant aptitude / selection tests), there is a complex career progression that includes type-specific training and then assignment to a squadron that operates a particular type of aircraft. Pilots who have trained on one type can't be randomly 'assigned' to a different aircraft without a lot of extra training.

Similarly, in the British Army, all soldiers go through basic training and then choose a particular military 'trade' (e.g. infantry, armour, artillery, etc). They receive trade-specific training before joining a relevant unit. Some regiments even have 'father and son' social events where an officer joined the same regiment that his father did.

The Royal Navy has similar career personal-choice progression routes for weapons officers, engineers, etc.