The heavies were, by and large, designed with a belly turret - a retractable "dustbin"- going back to the likes of the Handley Page Heyford
The Whitley had one too.
Even if installed, they were often removed, because they were heavy & when extended, caused a lot of drag, thus degrading performance.
The crews' choice was to fly higher & faster, especially during the early war at night, when no effective Luftwaffe nightfighter force existed.
It was only once this force was developed and especially when the Schräge Musik was in service, when the lack of a belly gun was looked upon as being an issue.
Some crews fixed up a hand-held, downwards-firing .5 to cover this blind spot.