Following World War 2 (1939-1945) and coinciding with the rise of the jet age, many American aviation companies fought one another for the limited amount of research-minded contracts becoming available. As was the case with the United States Air Force (USAF), the United States Navy (USN) also enacted several aircraft research programs to uncover the secrets of flight (and overcome its many dangers) for near-future applications. The first USN supersonic fighter, the Grumman F11F "Tiger", was born from a line of ever-evolving aircraft programs like those that produced the Douglas D-558-II "Skyrocket" rocket-powered, supersonic research plane. A Curtiss attempt set to rival the D-558-II was the ultimately abandoned "Proposal II".
The primary research role of Proposal II was to collect data on both subsonic and supersonic flight speeds and their relation to controlling surfaces, structure, and engines.
Proposal II was the second portion of what were two different research aircraft proposals offered by Curtiss - the first recognized simply as "Proposal I". Proposal I certainly looked the part of research aircraft as it was built along an extremely slim, needle-nosed fuselage with thin swept-back wing mainplanes and a "T-style" tail unit. Power would come from Curtiss-manufactured rocket motors. Proposal II was the more drastic of the two in that it showcased a more compact airframe designed in the mold of a three-finned "arrowhead" with the single-seat cockpit set at the base of the vertical tail fin. Wings were mid-mounted along the sides of an oblong fuselage which housed the necessary fuel stores, engines, cockpit, avionics suite, and data-collecting equipment. At front was a sole circular air intake which aspirated the twin engine layout. At rear were two exhausts ports set under and to either side of the tail fin. The arrowhead approach negated the use of horizontal tailplanes and was largely reminiscent of a German design during World War 2 - the Lippisch P.13A interceptor of 1944. This program netted a few small gliders for testing before the end of the war. The finalized interceptor product was to be powered by a unique coal-burning arrangement (the P.13A lived on in post-war testing as the "DM-1" experimental glider for NACA - forerunner to today's NASA).
Power for Proposal II was to come from 2 x Wright ramjet engines providing 6,000 lb of thrust each. An auxiliary Curtiss rocket was to be slung under the fuselage for an added 4,000 lb of thrust - primarily used for initial acceleration. Once the fuel for the auxiliary rocket was expended, the unit would be jettisoned away from the craft and left to fall on its own under the power of a parachute for recovery and reuse in future launches.
In both the Proposal I and Proposal II designs, it was detailed that a Boeing B-29 Superfortress would serve as the launch ship - meaning that the test aircraft would be carried aloft under the power of the converted four-engine bomber (the test vehicle sat atop the bomber's fuselage spine) and released around 30,000 feet. The test aircraft would then operate under its own power once launched. A tricycle undercarriage was fitted to allow the aircraft to glide back down after completing its tests. This sort of delivery method was not wholly uncommon to the period - early rocket planes like the Bell X-1 and North American X-15 were both air-launched vehicles and several USAF "parasite" fighter projects envisioned a similar "mothership" approach for an effective self-defense capability for high-flying bombers.
Dimensions for Proposal II included a wingspan of 21 feet, 6 inches, an overall length of 30 feet, and gross weight of 10,350 lb with the auxiliary rocket installed.
From its estimates, Curtiss engineers claimed a maximum speed of 747 miles per hour at 2,000 feet though for only 60 seconds. 975 miles per hour could be reached at 10,000 feet though only held for 10 seconds. The highest speed estimates reached 1,900 miles per hour at an estimated 50,000 feet of altitude.
The optimistic Proposal II eventually fell under criticism during its Navy review. It was believed that, despite its aerodynamically efficient arrowhead design, the aircraft would struggle to reach its intended supersonic speed - primarily due to the limitations of ramjets of the day which were only able to develop the needed power on their own once the aircraft itself had reached a speed of at least 1,205 miles per hour. Even the prospect of having the test aircraft dive to achieve the desired results was not enough to convince the Navy on the merits of Proposal II. Several of the Proposal II's project goals were also being met at the time by other developmental USN aircraft already in use. As such, the Curtiss Proposal II was dropped from any real, further consideration by the United States Navy.
The Douglas D-558-II went on to have a successful test career for the United States Navy and three were built. It reached a speed of Mach 2 on November 20th, 1953.