You have to set a key in your control options to arm your bombs.
Also, before spawning in the mission you have to select a correct detonator. There are high altitude fuses and low altitude/dive bombing fuses. The low alt detonators need a smaller amount of "bomb flight time" to arm and also have a detonation delay, which means you can drop from low altitudes and the bomb will explode with a delay to let you get away from the blast.
So after you set a key to arm the bombs in the options, in short you need to :
1) Go to the loadout screen and select a low altitude detonator (the one that has the default 14 second delay).
2) Fly the mission.
3) Arm the bombs.
4) Execute your attack and drop the bombs.
