As long as you have enough altitude it's perfectly possible to pick up speed in a vertical or near-vertical dive in a fixed wing aircraft then convert that vertical speed into forward speed.
Red bull recently did a stunt where they landed a fixed wing plane on a skyscraper helipad and then took off again.
Since the helipad was small, the pilot wasn't able to gain much speed before leaving the helipad, so he dived picked up speed and then pulled out of the dive.
The plane was heavily customised for the stunt, a more normal plane would likely need substantially more altitude to acheive a similar feat.