The Defiant or Boulton Paul Defiant was a bit of a mixed bag. It was at the cusp of the old RAF from WWI and the interim years. It had the old conservative design goals on a more modern platform which compromised its effectiveness.
https://en.wikipedia.org/wiki/Boulton_Paul_Defiant 
It was modern following the blueprint of a single wing with a liquid cooled engine that quickly turned into the Hurricane and Spitfire. Yet it had that hangover of old thinking, a rear gunner and turret but no forward firing guns. The told theory of one flying/watching and one firing.
It did not have the speed to engage in equal combat with the ME109 and so became a bit of a challenge for the squadrons equipped with it. As you say a pilot and rear gunner flying one of these needed greater bravery. It got moved to night use against the german bomber streams where it became less vulnerable.
All airforces had these sort of planes. For the German air force the Stuka was almost a death sentence towards the end of the air war due to limited speed and armour.
