# fields.py
from django import forms

class RequiredNullBooleanField(forms.NullBooleanField):
    def clean(self, value):
        value = super(RequiredNullBooleanField, self).clean(value)
        if value is None:
            raise forms.ValidationError("This field is required.")
        return value


# forms.py
from django import forms
from project.fields import RequiredNullBooleanField

class MyForm(forms.Form):
    question = RequiredNullBooleanField(label="Have you ever travelled in an airplane?", 
                                        widget=forms.RadioSelect(choices=[(True, "Yes"), (False, "No")]))