Learn Russian: Мы летим в город.

Летим comes from the unidirectional verb лететь. We use it because the sentence describes a single, specific trip happening right now in one direction (towards the city). If you wanted to say 'we fly to the city every week' or 'we fly around', you would use the multidirectional verb летать (мы летаем).

Is город in the accusative case here, even though it has not changed its ending?

Exactly! When talking about motion towards a destination, the preposition в (to/into) must be followed by the accusative case. Because город (city) is a masculine inanimate noun (an object or place, not a living being), its accusative form looks exactly like its basic dictionary form.

I learned that verbs ending in -еть usually have -ем endings for 'мы'. Why is it летим?

Great catch! While many verbs ending in -еть belong to the first conjugation and take -ем endings, лететь is an exception. It belongs to the second conjugation, which is why it takes the -им ending for 'we' (мы летим).

How should I pronounce в город together?

In Russian, short prepositions like в are pronounced as if they are part of the next word. You should say it without any pause, like a single word. Since the letter г is a voiced consonant, the в keeps its normal 'v' sound.