Tweak of Backbone.extend to allow deeper inheritance for properties such as `defaults` and `options` and another property you also want extended.

backbone.inherits.js

/* global _, Backbone */
(function(_, Backbone){
    // Helper function to correctly set up the prototype chain, for subclasses.
	// Similar to `goog.inherits`, but uses a hash of prototype properties and
	// class properties to be extended.
	Backbone.deepInheritableProperties = ['options', 'defaults'];

	var inherits = function(protoProps, staticProps) {
		var parent = this;
		var child;

		// The constructor function for the new subclass is either defined by you
		// (the "constructor" property in your `extend` definition), or defaulted
		// by us to simply call the parent's constructor.
		if (protoProps && _.has(protoProps, 'constructor')) {
			child = protoProps.constructor;
		} else {
			child = function(){ return parent.apply(this, arguments); };
		}

		// Add static properties to the constructor function, if supplied.
		_.extend(child, parent, staticProps);

		// Set the prototype chain to inherit from `parent`, without calling
		// `parent`'s constructor function.
		var Surrogate = function(){ this.constructor = child; };
		Surrogate.prototype = parent.prototype;
		child.prototype = new Surrogate();

		// Add prototype properties (instance properties) to the subclass,
		// if supplied.
		_.each(Backbone.deepInheritableProperties, function(key){
			protoProps[key] = _.defaults({}, child.prototype[key], protoProps[key]);
		});

		if (protoProps){ _.extend(child.prototype, protoProps); }

		// Set a convenience property in case the parent's prototype is needed
		// later.
		child.__super__ = parent.prototype;

		return child;
	};

	// Set up inheritance for the model, collection, router, view and history.
	Backbone.Model.inherits = Backbone.Collection.inherits = Backbone.Router.inherits = Backbone.View.inherits = Backbone.History.inherits = inherits;
})(_, Backbone);